You are claiming off your friends insurance, which is not the same as suing him directly, and to be fair if he is claiming off his insurance for the van in any case, then it is not going to make any real difference.
Speak to a specialist personal injury solicitor (don't deal with a claims management firm) who will send off the letter of claim to your friend who simply passes it onto his insurance company to deal with.
Given your injuries, don't expect your claim to settle overnight though, you are probably looking at a couple of years probably more before your case settles.