I have just spoken to an insurer from compare the market and they said all I would need to do if I converted to 4WD is click "engine modification" which ahs increased my premium by about £20 hahaha so it's all good! I've asked for it in writing as well to cover my own ass! 
Just a thought, but are there any vehicle inspection requirements or tests (similar to IVA for imported or kit cars) if substantial modifications are made to a car, such as converting a GTD to a 4WD GTI?
Is converting a FWD car to 4WD really considered to be an engine modification and not a drivetrain modification? Insurance aggregator sites such as Compare the Market, Moneysupermarket and the like are geared to writing predominantly large volumes of standard, unmodified insurance risks, with provision to cover some of the more straightforward modifications; e.g. a different set of alloy wheels. Insurers with products on Compare the Market etc. aren’t really interested in heavily modified cars - that’s not their area of specialisation - so may not have the technical know-how to handle this business.
With the modifications you’re planning, I think it’d be advisable to speak to either an insurance broker who specialises in placing (heavily) modified cars with specialist insurers who regularly write these types of risk, or speak direct to a specialist insurer of modified cars. They will have staff who are technically and mechanically minded and who’ll understand and fully appreciate the modifications you’re planning. That way, you should have the confidence of knowing you’re fully covered, and not just relying on your policy documents showing your car has ‘engine modifications’ with no mention of it being converted from FWD to 4WD.