I'm afraid that you are mistaken. This is England and you are not required to put up sign or anything at all like that.
There are two different processes depending on if the land is registered or not (almost all houses in England & Wales are registered).
Presuming that the land is registered then the squatter must occupy the land for ten years. They can then make an application to the Land Registry to claim the land.
Land Registry send a notice to the registered owner. This notice then gives the owner 13 weeks in which to reply. The owner can agree to give up the land or can oppose the application.
If the owner opposes the application then the Land Registry writes back to the other person saying don't be such a CF, you aren't getting the land.
If the owner doesn't reply within those 13 weeks then the other person gets the land.
So, as long as the owner opposes any application in time then the other person cannot gain possession (usually).
However, there are certain circumstances when the squatter may gain the land even if the original owner opposes.This is where the squatter reasonably believed that the land belonged to him/her.
This is laid out in paragraph 5(4)(c) of Schedule 6 to the Land Registration Act 2002:
(c) for at least ten years of the period of adverse possession ending on the date of the application, the applicant (or any predecessor in title) reasonably believed that the land to which the application relates belonged to him, and
An example of this might be where there are neighbours and after many years they find out that the boundary is in the wrong place, but they had thought that it was in the right place. In that situation, the court is likely to say that the wrong boundary line is now the actual boundary.
Or, in the case of the OP, if the neighbour had genuinely believed that the bit of land where they built the driveway was indeed part of their property.
In fact, there was a case that went all the way to the Supreme Court last year on just this point, Brown v Ridley [2025] UKSC 7.
The Supreme Court said that if a neighbour genuinely believes for a period of ten years that a piece of land, which is adjacent to their own, is theirs and they treat it as such then they will be entitled to be registered as the new owners.
In Brown v Ridley, they were neighbours and a previous owner of the Ridley's house had enclosed some of the garden belonging to the Brown's house.
The Ridley's were unaware of this. They obtained planning permission to build a new house that was partly on the land that belonged to the Browns, which led to the court case.
.
This is totally different from where a person deliberately takes control of a piece of land knowing that it is not theirs.