The withdrawal period is a very good point and one I have already done some thinking about. Luckily the medication she is on has a withdraw period of only 5 days and at the moment she isn't having it that often. Currently, it's been about two weeks since she last had some. So assuming she doesn't have a bad turn this should be fine. However, of course if she did take a turn and was suffering I would provide pain relief and cancel the abattoir.
In regards to whether or not she would be fit to travel, I don't think this will be an issue either. She will be able to comfortably make it on and off the trailer and can weight bear on all four legs. Part of my reasoning for deciding to do this now is that I think probably in the near future she won't be able to do this and would have to be euthanized on the farm instead which would be a waste. Our vet came out a few weeks ago and we discussed this briefly then and he agreed this was the best option and didn't seem to have any worrys about welfare, fitness to travel etc.
You can't "finish" an old pig only "fatten" it and generally speaking it goes into processing - sausages, pies, pate etc. It will possibly have a stronger flavour and wont be prime so if you do attempt joints they will need more cooking. You could think about mince or diced pork for longer cooking.
Not all abattoirs will/can take "big" pigs as they wont fit in the dehairing machine so will be skinned.
Sounds similar to mutton then? I didn't realise not all abattoirs take big pigs so thank you for making me aware of that I will definitely make sure to check with mine!
Thanks again guys, you've been really helpful!