Use RFA java to subscribe to VOD.L My application currently look for FID RDN_EXCHD2,GV5_TEXT,IRG_EXID,IRGXID to figure out at which exchange a trade has occurred.
Here I provide two trade update received.
Message 1:
In  the following trade update message, none of the above FIDs are available. By looking at the data, it appears 8519/TRD_P_XID: XOFF, or 1036/GV6_TEXT: XOFF could do the job. 
But Data Model Discovery for LSE does not mention RTD_P_XID at all: https://refinitiv.fixspec.com/stack/specs/elektron/emea/united-kingdom-london-stock-exchange-lse/6-1-8/1885-london-stock-exchange-lse-equities-etf-and-orb-ertequities 
Msg Type: MsgType.UPDATE_RESP
    Msg Model Type: MARKET_PRICE
    Indication Flags: DO_NOT_CONFLATE
    Hint Flags: HAS_RESP_TYPE_NUM | HAS_SEQ_NUM
    SeqNum: 43598
    RespTypeNum: 2 (TRADE)
    Payload: 360 bytes
        FIELD_LIST
            FIELD_ENTRY 8519/TRD_P_XID: XOFF
            FIELD_ENTRY 1069/CONDCODE_2: 
            FIELD_ENTRY 975/ACT_FLAG1: T
            FIELD_ENTRY 32694/LTNOV_UNS: 1143362553.48
            FIELD_ENTRY 12840/ELIGBL_TRD: N   (2)
            FIELD_ENTRY 14266/SALTIM_NS: 15:58:43
            FIELD_ENTRY 1028/GV1_DATE: 17 AUG 2020
            FIELD_ENTRY 9065/OFBKTIM_MS: 57523000
            FIELD_ENTRY 9302/SRC_ES_DT: 17 AUG 2020
            FIELD_ENTRY 1021/SEQNUM: 254595
            FIELD_ENTRY 1068/CONDCODE_1: T
            FIELD_ENTRY 8537/OFFBK_CD_N: 
            FIELD_ENTRY 77/NUM_MOVES: 8817
            FIELD_ENTRY 7597/OFFBK_TNOV: 1385.72946
            FIELD_ENTRY 1034/GEN_VAL10: 66.97
            FIELD_ENTRY 3854/SALTIM_MS: 57523000
            FIELD_ENTRY 1036/GV6_TEXT: XOFF
            FIELD_ENTRY 5304/OFFBK_DATE: 17 AUG 2020
            FIELD_ENTRY 32743/ACVOL_UNS: 35442039
            FIELD_ENTRY 14272/OFBKTIM_NS: 15:58:43
            FIELD_ENTRY 1067/EXCHTIM: 15:58:43
            FIELD_ENTRY 32/ACVOL_1: 35442039
            FIELD_ENTRY 3853/TRDTIM_MS: 57523000
            FIELD_ENTRY 3900/TRADE_ID: 1043414762576855152
            FIELD_ENTRY 1033/GEN_VAL9: 11707809
            FIELD_ENTRY 3876/OFFBK_VOL: 11707809
            FIELD_ENTRY 379/SALTIM: 15:58:43
            FIELD_ENTRY 18/TRDTIM_1: 15:58:43
            FIELD_ENTRY 8643/OFFBKTRDID: 1043414762576855152
            FIELD_ENTRY 2381/GV1_CURRCY: GBp (2008)
            FIELD_ENTRY 14312/SRC_ES_NS: 15:59:31:910:170
            FIELD_ENTRY 16/TRADE_DATE: 17 AUG 2020
            FIELD_ENTRY 6582/TRVOLOFFBK: 9653517
            FIELD_ENTRY 956/QTE_CNT2: 118.042
            FIELD_ENTRY 4109/OFFBK_PRC: 118.44
            FIELD_ENTRY 3404/VWAP: 118.04200
            FIELD_ENTRY 6/TRDPRC_1: 118.44
            FIELD_ENTRY 4756/LSTSALCOND: 
            FIELD_ENTRY 4465/TRD_IND_1: T
            FIELD_ENTRY 100/TURNOVER: 4183.65782
            FIELD_ENTRY 14/PRCTCK_1: Þ (1)
            FIELD_ENTRY 178/TRDVOL_1: 9653517
            FIELD_ENTRY 13432/TR_TRD_FLG: RO    (7)
            FIELD_ENTRY 8582/MMT_CLASS: 46-------P----
            FIELD_ENTRY 5290/PCT_OB_VOL: 66.966
            FIELD_ENTRY 6842/OFFBKSEQNO: 254595
            FIELD_ENTRY 32741/TRNOVR_UNS: 4183657816.09575
            FIELD_ENTRY 995/GEN_TEXT16: 
            FIELD_ENTRY 3859/SEQ_NO: 254595
            FIELD_ENTRY 6914/OF_NUM_MOV: 883
Message 2:
In the following message, we have 1035/GV5_TEXT: BTE, 1036/GV6_TEXT: LSE, 8519/TRD_P_XID: BTE  
my existing logic will pick LSE as the trade's exchange, but maybe BTE should be the correct value.
Msg Type: MsgType.UPDATE_RESP
    Msg Model Type: MARKET_PRICE
    Indication Flags: DO_NOT_CONFLATE
    Hint Flags: HAS_RESP_TYPE_NUM | HAS_SEQ_NUM
    SeqNum: 19838
    RespTypeNum: 2 (TRADE)
    Payload: 521 bytes
        FIELD_LIST
            FIELD_ENTRY 6/TRDPRC_1: 117.7600
            FIELD_ENTRY 178/TRDVOL_1: 619
            FIELD_ENTRY 32/ACVOL_1: 22068305
            FIELD_ENTRY 77/NUM_MOVES: 13527
            FIELD_ENTRY 14/PRCTCK_1: Þ (1)
            FIELD_ENTRY 1021/SEQNUM: 13527
            FIELD_ENTRY 3404/VWAP: 118.0171
            FIELD_ENTRY 11/NETCHNG_1: -0.9200
            FIELD_ENTRY 56/PCTCHNG: -0.775
            FIELD_ENTRY 1067/EXCHTIM: 15:17:02
            FIELD_ENTRY 18/TRDTIM_1: 15:17
            FIELD_ENTRY 996/GEN_VAL1: 117.7600
            FIELD_ENTRY 100/TURNOVER: 26.0444
            FIELD_ENTRY 16/TRADE_DATE: 17 AUG 2020
            FIELD_ENTRY 1665/GN_TX20_1:                     
            FIELD_ENTRY 41/CONTR_MNTH:     
            FIELD_ENTRY 1666/GN_TX20_2:                     
            FIELD_ENTRY 1036/GV6_TEXT: LSE   
            FIELD_ENTRY 1668/GN_TX20_4:                     
            FIELD_ENTRY 1035/GV5_TEXT: BTE  
            FIELD_ENTRY 1667/GN_TX20_3:                     
            FIELD_ENTRY 1037/GV7_TEXT: LSE   
            FIELD_ENTRY 1038/GV8_TEXT: LSE   
            FIELD_ENTRY 1051/GV2_DATE: 17 AUG 2020
            FIELD_ENTRY 1039/GV9_TEXT: LSE   
            FIELD_ENTRY 1669/GN_TX20_5:                     
            FIELD_ENTRY 374/IRGCOND:     (0)
            FIELD_ENTRY 2320/ALIAS: BTE
            FIELD_ENTRY 3900/TRADE_ID: 254078890549            
            FIELD_ENTRY 4465/TRD_IND_1:       
            FIELD_ENTRY 8519/TRD_P_XID: BTE       
            FIELD_ENTRY 7598/LST_CLN_PR: 117.7600
            FIELD_ENTRY 8539/LCLNPR_SRC: BTE       
            FIELD_ENTRY 6585/CAN_COND:     (0)
            FIELD_ENTRY 7/TRDPRC_2: 117.7400
            FIELD_ENTRY 10/TRDPRC_5: 117.7000
            FIELD_ENTRY 9/TRDPRC_4: 117.7000
            FIELD_ENTRY 8/TRDPRC_3: 117.6800
            FIELD_ENTRY 3402/TRDVOL_5: 1000
            FIELD_ENTRY 3401/TRDVOL_4: 1983
            FIELD_ENTRY 3399/TRDVOL_2: 166
            FIELD_ENTRY 3400/TRDVOL_3: 513
            FIELD_ENTRY 8520/TRD_P_XID2: LSE       
            FIELD_ENTRY 8521/TRD_P_XID3: LSE       
            FIELD_ENTRY 8522/TRD_P_XID4: LSE       
            FIELD_ENTRY 8523/TRD_P_XID5: LSE       
            FIELD_ENTRY 4466/TRD_IND_2:       
            FIELD_ENTRY 4468/TRD_IND_4:       
            FIELD_ENTRY 4469/TRD_IND_5:       
            FIELD_ENTRY 4467/TRD_IND_3:       
            FIELD_ENTRY 3854/SALTIM_MS: 55022688
            FIELD_ENTRY 8582/MMT_CLASS: 12-------PH---  
            FIELD_ENTRY 13432/TR_TRD_FLG: OB    (1)
            FIELD_ENTRY 14266/SALTIM_NS: