How quick they are to deliver
Very quick, if you order from the Showcase. If you want a custom order it'll take a few weeks, I believe.
How reliable/customer friendly they are
Totally reliable, and they respond quickly to emails. I personally haven't dealt with them by phone, but I understand they're very helpful indeed.
Is it really as easy as screwing all parts together or is there an art to it?
You'll still have to drill a few holes - for the scratchplate and backplate, jack socket and tuners. If you buy a neck with a pre-installed nut, it'll be perfectly playable but from what I've seen it might be even better after a little attention to the frets and setup.
I assume I would be able to pull a decent strat together for around £750.00.
Depends on the options you choose, but don't forget to factor in shipping charges, VAT and import duty. And the terrible exchange rate at the moment!