Don't damage the frame with the key of the bolt. If it is already spinning then you might already have damaged the frame.

The bolt is probably knackered if you can't get it out easily -but you might be able to save the nut (A replacement will be expensive for the R-branded nut).

I'd try and cut the middle of the bolt with dremel if it is a seatpost bolt and then pull it out and put it in a vise to remove and save the nut after putting in some penetrating oil.

If you can't get at the fastener to cut it off in the middle I'd try using a vise grips to grip the round edge. If that isn't working or you are afraid of scratching the frame/paint then you can try and drill the end off carefully, or use a dremel tool to put a notch in it so that you can put a flat screwdriver in it.

The bolt isn't very expensive and if it is turning then either the key is busted already or the frame is stripped where the key slots in. You're probably going to have to ruin the bolt to get it out even if that is as little as marking it up with vise grips marks from grabbing it.

But don't ruin the Raleigh nut as they are expensive and sometimes hard to source depending on the thread pitch (the old kottered crank nuts are really a PITA to source). You can replace the keyed bolt with an Allen-head or Torx-head bolt that matches the thread on the nut and It'll still look nice (and be more theft-resistant in the process.