What sort of steel? Does it look at first like ordinary copper pipe, 15mm or 22mm or equivalent, which will be the thin frightening stuff, which can be plated so it's solderable?
Or is it "barrel" which is thick and heavy with threads cut on its ends which go into bulky screwed connections? "Half inch barrel" - the size rads would normally have, is about 3/4" outside diameter. Barrel can be galvanised or "black".
Yours is a mixed system, which is worse than all steel, because of the increased scope for electrolytic corrosion.
Any old pipe, copper or more likely steel, most especially the thin steel one, can have pin-holes covered in paint or oxides (Cu or Fe) which is stopping them leaking.
One in ten or so old copper systems leaks enough to need attention when powerflushed with chemicals. Some say it's better to use stronger ones (eg citric/phosphoric acids) to really find any future trouble spots sooner rather than later. Often the holes are in the middle of radiators, covered by paint.
Leaks often don't appear until later, after the new boiler's gone in.
If you get a leak, it isn't usually a disaster, you just get a puddle, and you fix it. Nobody would indemnify the thin steel though. If you get slight weep, eg at screwed joints, you can use internal leak sealer. Ask BG for their terms; small companies aren't in the business of selling insurance. It's your house, a lot of unknowns and your risk, unless you pay enough to cover all eventualities.
You don't have to use any chems, fit a filter and magnetic trapper in the return to protect the boiler and you'll probably be ok - these are new so nobody will indemnify on the strength of their use. It's combis which are particularly susceptible to clogging up with system grot. If use a cast iron thing like a Potterton Profile 50 (<£500), they don't care about it because the waterways are huge. Non condensing so will cost you plenty in gas.
If it's part barrel, better out than in, rads don't last forever either. It might be reasonable for you to keep it, but if I were buying the place I'd take the cost of replacement off the house price, no questions.
If you flushed with plain water or gentle chems you could pressure test it but rad valve stems etc do lose a tiny bit so you wouldn't really know where you were. The tiny leak could be from a pipe in the middle of a ceiling, about to get much worse. And it might very well hold up 3 bar cold, then lose it when hot, or after a week under gravity at only 5 meters head (0.5 bar).
Your next boiler might last say 15 years (Profile nearer 25?). If your rads & steel pipes are already 35 they'll certainly be due, most likely overdue, for replacement then.
If you want a full metallographic analysis, all about ion capture, pitting cell morphology, intergranular corrosion development, passivation maintenance, where the steel was made, and things, cut it up and send me £5k and some samples. It might be interesting, though probably not, and I still couldn't say if it was going to leak tomorrow.
Unless you`re Central London 
