This is going back a while now so this might have changed, but I seem to recall a similar problem when I was trying to join – they told me I could still join by providing a scan of a voided cheque from my UK bank account.
Partly because I thought this was a bit of a weird and out-dated way of doing things and partly because I am disorganised and forgetful (!) I never joined.
If any other UK people reading this have had any joy with joining SoundExchange I’d love to hear about it (and please let me know if I’m completely incorrect regarding the above!)