A while back there was a discussion about accepting multiple currencies. This is something that I’ve set up for DataTables sales as well, but, as @rachelandrew indicates in the earlier thread, you need extra information in order to be able to do UK VAT accounting correctly.
To that end I wrote an API for use internally - a backend service consumes the data published by HMRC and stores it in a database that the API can query. I use this for the majority of my invoices now, and I thought some other bootstrappers in the UK might be able to benefit from it.
I hope one or more of you find it of some use!