The license would be the PPL(A)-"Add on".
I believe it requires 35hrs flying, some of the cross country is waived.
You get credit for the written, though would need to do the Oral during the Practical test.
It is also applicable if you have your CPL (i.e. you can turn a CPL(H) to CPL(A))
Don't have any links handy, but I have seen quite a few schools, who advertise this. (Those who advertise tend to be schools that do both Fixed and Heli training, though any fixed wing place should be able to do it - though I'd personally go with a school that has done it specifically)
Here is a link to
North East Helicopters Add-on package description, though it discusses PPL/CPL(A)->(H)
(I don't know them, just one I had seen in the past, and using here as an example)