It’s relatively straightforward, in my (limited) experience, and it shouldn’t cause any legal problems with selling your home in the future, provided you register the new boundary with the land registry, and have the conveyancing done properly (that may mean paying a solicitor). It might however reduce the value or desirability/saleability - ask a couple of local estate agents. They will also give you advice on value. Your neighbour should also do this.
We bought a smallish piece of garden land from a neighbour, and agreeing a value was an interesting process, because he wanted to sell at residential prices (which would have been about £10k) and we wanted to buy at agricultural land prices (c£2k). We compromised somewhere in the low middle, and we paid the legal costs, (just one solicitor, as there was no dispute), a surveyor to draw up a new boundary plan, and a few quid for the Land Registry fee. There were no mortgage companies involved; I understand there is a little additional work (and costs) if there are.
It was very quick, weeks not months. It did not reduce the value of his property.. There was a small increase in the value of our house, but our neighbour disregarded that as he was planning to move, so didn’t care if we built on it. (We haven’t, we’ve made a small meadow, which annoys some other neighbours more than an extension would!)
You need to factor any legal costs, and potential reduction in the value of your house, and - equally important - any uplift in the value of your neighbours property, into the price you’d sell the land for.
And once you’ve sold, you have very little control over what happens on the land.