You have a few choices:
- Go for temp insurance. As already discussed, likely to be expensive.
- You could re-insure the truck and then cancel the insurance when you've sold it. You should be able to claim something back for the unused period, but there'll be an admin fee, and/or you won't get a pro-rated amount back.
- You could leave re-insuring to the last minute, and then (hopefully, if the truck is sold) cancel within 14 days. In this case you should get back the entire premium, except any pro-rated amount to cover the period actually covered. However, there shouldn't be a fee. (I say leave to the last minute because I've never been clear if the 14 day cooling off period starts from when you buy the policy, or when the policy starts - so leave till the last minute to get the maximum 14 days!)
- SORN the truck and don't reinsure. This'll make a test drive a problem

shhh

and trickier for the buyer to pick up (he'll need to insurance and have a print insurance of the tax plus the MOT cert to be able to tax at the post office). (Assumes you can leave off the road.)
- Don't SORN the truck, don't re-insure

eusa-naughty

. I've seen suggestions that it takes a month or two for the DVLA to pick up on this, but I personally wouldn't risk it. (Assumes you can leave off the road.)