It's a one-time thing you have to go through. It's why I'm extremely glad I have my own domain which doesn't care who my ISP is.
I'd be inclined to ask Pipex if you can keep it "live" for three months or so - but from my experience with them, I wouldn't be over-hopeful.
When I moved from the last "ISP" address I used, I went through all the e-mails in my "keep" folders (isn't Thunderbird wonderful) and wrote to all the relevant ones. Some ignored the change of address anyway, and some of those ended up writing to me (snail-mail) asking for my new address.
The other thing I did, with my own domain and unlimited forwarders: each "seller" got his own address. So, amazon@keef, bt@keef, maplin@keef, and so on. One firm (that shall remain nameless) sold my e-mail address, and the spam started. I changed the forwarding address of that e-mail address to the offending firm's.