The routeing guide does not say what to do in this case, but if you take the most reasonable idea, which is to say all routes Swindon to Holyhead as valid, via London is not a valid route. Also not your ticket doesnt have the maltese cross...
A routing points: [Routing Point SWINDON (WILTS)]
B routing points: [Routing Point SHOTTON]
ROUTING Station SWINDON (WILTS) TO Station Holyhead
14 routes found
route [Map MW, Map LC]
Map MW to Map LC via [Routing Point LIVERPOOL GROUP, Routing Point STOKE ON TRENT, Routing Point COVENTRY, Routing Point SHOTTON, Routing Point HUNTS CROSS GROUP, Routing Point WOLVERHAMPTON, Routing Point EARLESTOWN, Routing Point WARRINGTON GROUP, Routing Point WREXHAM GROUP, Routing Point SMETHWICK GROUP, Routing Point CREWE, Routing Point CHESTER, Routing Point WALSALL GROUP, Routing Point BIRMINGHAM GROUP, Routing Point RUNCORN GROUP, Routing Point HUYTON, Routing Point LICHFIELD T V, Routing Point NUNEATON, Routing Point HOOTON, Routing Point STAFFORD GROUP, Routing Point TAMWORTH]
route [Map MW]
route [Map BL, Map CC]
Map BL to Map CC via [Routing Point BIRMINGHAM GROUP, Routing Point BARNT GREEN]
route [Map CE, Map BC, Map CH]
Map CE to Map BC via [Routing Point BIRMINGHAM GROUP, Routing Point SMETHWICK GROUP]
Map BC to Map CH via [Routing Point HOOTON, Routing Point SHOTTON, Routing Point CREWE, Routing Point WREXHAM GROUP, Routing Point CHESTER]
route [Map CE, Map CC]
Map CE to Map CC via [Routing Point BIRMINGHAM GROUP, Routing Point BARNT GREEN, Routing Point SMETHWICK GROUP]
route [Map BD, Map CH]
Map BD to Map CH via [Routing Point SHOTTON, Routing Point WREXHAM GROUP, Routing Point CREWE, Routing Point CHESTER]
route [Map CE, Map BC]
Map CE to Map BC via [Routing Point BIRMINGHAM GROUP, Routing Point SMETHWICK GROUP]
route [Map BL, Map BC, Map CH]
Map BL to Map BC via [Routing Point BIRMINGHAM GROUP]
Map BC to Map CH via [Routing Point HOOTON, Routing Point SHOTTON, Routing Point CREWE, Routing Point WREXHAM GROUP, Routing Point CHESTER]
route [Map SB, Map CE, Map CC]
Map SB to Map CE via [Routing Point CARDIFF CENTRAL, Routing Point SWINDON (WILTS), Routing Point NEWPORT (S.WALES), Routing Point PONTYPRIDD, Routing Point CHIPPENHAM, Routing Point BRISTOL GROUP, Routing Point BATH SPA, Routing Point BRIDGEND, Routing Point SWANSEA GROUP]
Map CE to Map CC via [Routing Point BIRMINGHAM GROUP, Routing Point BARNT GREEN, Routing Point SMETHWICK GROUP]
route [Map SB, Map CE, Map BC, Map CH]
Map SB to Map CE via [Routing Point CARDIFF CENTRAL, Routing Point SWINDON (WILTS), Routing Point NEWPORT (S.WALES), Routing Point PONTYPRIDD, Routing Point CHIPPENHAM, Routing Point BRISTOL GROUP, Routing Point BATH SPA, Routing Point BRIDGEND, Routing Point SWANSEA GROUP]
Map CE to Map BC via [Routing Point BIRMINGHAM GROUP, Routing Point SMETHWICK GROUP]
Map BC to Map CH via [Routing Point HOOTON, Routing Point SHOTTON, Routing Point CREWE, Routing Point WREXHAM GROUP, Routing Point CHESTER]
route [Map SB, Map CE, Map BC]
Map SB to Map CE via [Routing Point CARDIFF CENTRAL, Routing Point SWINDON (WILTS), Routing Point NEWPORT (S.WALES), Routing Point PONTYPRIDD, Routing Point CHIPPENHAM, Routing Point BRISTOL GROUP, Routing Point BATH SPA, Routing Point BRIDGEND, Routing Point SWANSEA GROUP]
Map CE to Map BC via [Routing Point BIRMINGHAM GROUP, Routing Point SMETHWICK GROUP]
route [Map BD]
route [Map MW, Map BC]
Map MW to Map BC via [Routing Point SHOTTON, Routing Point SHREWSBURY, Routing Point WOLVERHAMPTON, Routing Point WREXHAM GROUP, Routing Point SMETHWICK GROUP, Routing Point CREWE, Routing Point CHESTER, Routing Point WALSALL GROUP, Routing Point BIRMINGHAM GROUP, Routing Point HOOTON]
route [Map BL, Map BC]
Map BL to Map BC via [Routing Point BIRMINGHAM GROUP]
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So for example you could go Didcot to Birmingham to Crewe to Holyhead