The operator/pilot had probably looked at his charts and noted that the closest reporting point to Bristol was TUTON and then he could join UN862 to BHD where it the runs to DIN as does A25, they probably didn't realise that it was only an UAR.
The controllers at Bristol would (guessing here!) have the filed plan in front of them showing UN862 - However the London chaps and chapesses would be looking at an amended flight plan that would have been changed by those clever FPRS people to an EXMOR join hence the clearance given! Leaving the pilot totally confused I'm sure
