pipertommy
I don't think any UK legislation states what equipment you must use. It lists what you must carry.
Everything else is conjecture and perhaps wishful thinking.
Sometimes wholly plausible conjecture (like you are supposed to fly an ILS with an ILS receiver and not by dead reckoning) but at other times it is certain nonsense (like not allowed to use a certain type of GPS when IFR in Class G).
An "IFR GPS" is a specific category of GPS which you need to have if flying on BRNAV routes, above FL095 in Europe. In the UK these are all in Class A anyway.
As tmorris rightly says, you can legally use dead reckoning in IMC. That is about the least accurate and least safe way to do anything, so anything will be better.
You can use any GPS for primary navigation ("primary" is another term not defined in legislation but widely touted by amateur aviation lawyers) anytime.
So I think you worry too much. Get yourself the very best moving map GPS you can afford, make sure it comes with an external antenna which you can clip to a window (for a decent view of the sky) and enjoy yourself!
Around the UK, one can do most flying in Class G.