Put something akin to this in the context that handles your UK calls.
exten => _080XXXXXXX,1,Dial(SIP/${EXTEN}@tollfreeuk.com)If you dial from outside the UK and want people to call, for example 0044800xxxxxxx, you probably want something like
exten => _004480XXXXXXX,1,Dial(SIP/0${EXTEN:4}@tollfreeuk.com)If you need to dial 9 for an outside line first, you probably want something like this
exten => _9080XXXXXXX,1,Dial(SIP/${EXTEN:1}@tollfreeuk.com)