Do I file a flight plan
You have two choices. 1 - low level outside the airway system (and no gurantee of CAS access) in which case you don't prefile a flight plan, but if you blag access to CAS (which won't be the TMA) you will have effectively had a flight plan created for you when you get the 'Cleared to Enter' call, 2 - 'A Eurcontrol Route' that is a route submitted into the IFR system and compliant with the route structure. In this particular case something like
-EGMD N0123F090 DET N57 LAM EGTR0041
I simply flying along in the clouds at my own chosen height pretty much as if I was in VFR except I can't see the ground, staying in G airpace? If so, how do I separate myself from another IR aircraft doing the same?
IFR OCAS below the transition level is basically just 1000 ft above the ground within 5 miles. With regard to separation you can either use a radar service (such as Farnborough) or Big Sky (in the UK no civil aircraft has yet hit another in IMC)
can I land on it even though it has no ILS or NDB? Is there like a Minimum Decision Height for airports with no navaids or ILS's?
This is a little tricky. The rule is drafted so that you can only decend below 1000 ft in IMC if you are visual with the ground or approaching to land. Oddly it is not a requirement (in the UK) that you are using an approach. So it is not illegal (except to the extent it might be reckless) to land without following an approach in IMC
If not, can I do an ILS approach down to where I'm in sight of the ground at a close by airport (Northolt, for instance), and then fly low level over to Elstree?
Yes, but at 400 ft ceiling you will be hard pressed to get from Northolt to Elstree while remaining out of cloud and 500 ft above any person or structure and glide clear and built up areas ....
Is the GPS legal to do approaches to minimas on?
of course it is - GPS approaches have minimas just like all other approaches. You might have meant to ask, can I just make up an approach and use my GPS to navigate - and this is just as legal (or reckless) as making up any other approach.