We attend church while traveling because we like to, but marvelous side benefits are free history lessons & a sense of time travel, and meeting friendly locals and expatriates (there is almost always a coffee-tea-biscuits-chatting time after each service).
St Giles' in Edinburgh was mentioned upthread. We didn't have time to attend a service there, but I wish we had, as it is where John Knox preached. We did attend a service at "Met Tab" in London, Spurgeon's church, which was quite an experience – I don't think we've seen such diversity in a congregation anywhere else in the world. In December we attended a German/English advent carol service at University Church (St Mary) in Oxford; a friendly lady in our pew told us Wesley had preached his last C of E sermon in that pulpit, and C S Lewis had preached during WWII; and Dietrich Bonhoeffer's sister was among the German refugees who started a German-language congregation that has met there since the 1940s. In a small town in Cornwall we attended a service at the local Methodist chapel; it had been built to hold 700 and there were maybe 30 people in attendance, who insisted we stay for cake and tea afterwards and entertained us with local gossip. At a lovely C of E church near Reading, during coffee afterwards, we were told why 25% of the church was walled off – "the Reformation is still going on here," as 1/4 of the church still belongs to the Catholic Church (!) – and bombarded with helpful information about our upcoming trip to Cornwall.
ejl – best wishes as you plan your visit!