What's fishy about it? I don't know if you're aware, but Salisbury Cathedral attracts visitors from all over the world. Are you suggesting that two male Russians could not have a historical interest in architecture? They also wanted to go to Stonehenge, which, in case you haven't heard of it, is a World Heritage Site, thousands of years old.

I assume you haven't actually read up on it, because if you had, you wouldn't find tourists doing touristy things to be fishy, rather you'd think it was fishy that the Skirpals turned their phones off and drove out of Salisbury, before the "tourists" arrived in Salisbury. CCTV caught the Skirpals leaving Salisbury, but didn't see them return, even though they must have, because otherwise they couldn't have got poisoned.

The official story if fishy as fuck. The tourists' story is, well, reasonable.