Bike crash - what can I safely salvage from the old bike?

Would I use your wheels second hand knowing their history? No.

Consequences of failure are rather more than say the chainring being a bit out of shape and prone to dropping a chain.

If the rims are alloy, keep them. If they're carbon and if the tyres remained inflated then it's highly likely that the wheels are fine. Yes I would keep them and use them, perfectly happily. If there is no obvious mechanical damage to the rims it is very hard to see how they could possibly have been compromised. Everything else, if it works, keep it. 

"Could I reuse the wheels? Would you trust them? The front wheel is still basically true, but would you worry about unseen damage to the rim or hub? What about the bars or stem?"

Carbon: throw them away as underlying fiber layers may be damaged.

Alloy wheels; run your fingers around the brake tracks and feel whether the weld joint has separated. Remove tires, tubes and tape. Inspect the inside seam weld, and spoke holes.

Bar and stem and calipers - inspect for cracks/damage.

Bottom line if you feel they're damaged throw them away. Put yourself in the place of a buyer; how would you feel buying a damaged part that breaks while riding? I don't know the laws in the UK but here in the states you can get sued for liability, and the law will come after you too for knowingly selling damaged goods.

BTW, good to hear you're okay.

Mine torer went under a pickup a couple of years ago. All I salvaged from the old bike were the saddle (well-worn B17!), rear rack, mudguards, lights, bottle cages.

Frame was trashed, as were the handlebars, rear wheel, mechs and chainset (which took the brunt of the collision).

I didn't re-use the brakes (new bike has discs). I've kept the brake levers, brakes, and front wheel.

I scrapped the pedals - I couldn't be sure that the spindle on one or both might be slightly bent, mybe leading to knee troubles later if re-used. Ditto the stem (handlebars were bent so what impact was made on the stem?), bottom bracket, rear hub, becasue there was some impact which might have caused unseen damage.

Drivetrain should be all good. Give the shifters a go but I'm guessing they'll be trashed. Check the saddle rails for bends. Seatpost probably OK but depends on how close you were to minimum insertion (missus). Rear wheel will be fine but might be a challenge to get a matching front.

In summary: get LBS to price the repairs new-for-old and that should be enough to write the bike off and give you a box of spares for the shiny new one