I've been using Italia Leather straps for a long time. Unlike what the name suggests, they're actually made in California
They come in 2 widths (2.5" and 4") and 2 lengths (37-49" and 42-54") so they fall just a little short of what you're looking for.
They only sell directly though, and there will be import duties when having one shipped to the UK. They're expensive at around 100-110 USD per piece. That said, they often have sales (for returning customers, but the weblinks and discount codes to the sales can be used by anyone) like free shipping, buy-one-get-one-free or something along those lines. The most recent one was a 24 hour sale on April 6th, but I'm sure their next mailing will be along shortly . That's my one gripe with the company: they keep spamming your mailbox with deals. You can easily opt out, and I really should because I already have 8 at this point...