Hi ,
You picked one of my favourite decorations,timber and rads,
Ok, for your gloss "oil based" you want Sikkens Satura primer plus, your first coat and vechicle for your Undercoat and eventual gloss, but with this primer you dont really need a undercoat so go straight in the glossing using Sikkens Satura A/Z gloss, now these are oil based so the usuaul 16 hrs dring times between coats apply, but sir the finish is well worth it and these products are a little sxtra in cost but i promise you this is one of the very few oil based gloss systems left that stay white, and it will stay white out lasting any other oil based system on the market.
Radiators MSP, by BEDEC water based system but absoulutly brilliant for all surfaces and the whiteness is spectacular, again a little dearer but no other system on the market will out perform this paint.
I f you wish to go water based on your timber/trim then go with Sadolin Supadec, it comes in all sheens and is a superb paint again out performing everything else on the market in its catorgery, for this i would use a primer/undercoat such as wickes arcryilic, cheap but very effective,"german".
All the best and i hope these products find a way into your home has you will reap the benifits of a longer lasting fuller looking decoration for atleast 8/10 years without need of redecoration.
kind regards
chris