The IR represents a high internationally-standardised level of flying skill, hence they're presumably happy to do that quickly and easily.
The separate UK PPL will cost you somewhere around £40-£50 and will ensure you have a UK licence when you return, as none of us have a clue how things will progress over the next three years for UK/EASA co-operation, I'd strongly recommend getting that done, even if you have to revalidate it with a proficiency check when you return to Britain.