This is strange. I can do SEPA transfers with Intersango to Germany, and I sent and accepted SEPA transfer even inside Germany, so I can't see why Intersango should be able to switch from a domestic wire to a UK to UK SEPA, it is at least possible. Hm, how much do you need to transfer and do you need to transfer money often, I might open a polish GBP-Account next monday, which I could offer as a pass through.
It's much more expensive to do SEPA transfers to/from the UK. The typical charge is 17 GBP to send/receive a EUR transfers (both domestic and internationally).
On top of that there is a conversion from EUR-> GBP with a typical spread of 3%.
So in the end you can be talking 17GBP + 3% in bank fees alone.
Domestic UK bank transfers by contrast cost anything from free to 1GBP (the highest I've yet to see).