I think I have a slight leak on one of the exhaust manifold gaskets on my 4.0L TVR v8 engine (rover based). Replacing them is a pain in the bum but something I plan to do in the next few weeks for piece of mind...

However, am I likely to do any long term damage if I drive the car as it is? I had planned to do about 300 miles over to Wales... I can sort the gaskets there but I would like to enjoy the drive up

It will not hurt the engine, you may however gas yourself driving and if the leak is pointing towards something combustible have a fire to deal with.

And wear earplugs.