In reply to Baz P:
I think you're overimaging thhe headache. Just
1) get a domain for a few pounds a month or a single annual fee.
2) Email everyone on BT address
3) Pay BT fees for 3-6 months(if at all) with an auto-forward on/ [you could just do this for 6 months before paying for your own address]
4) Change all emails and contact details on a footer on your email, set-up an auto-responder, on your invoices, as a mailshot to all contacts, mass text to phone book etc and everyone will get the message
We migrated quite easily at work with a bridging gap