I know of nothing in BS 7671 that says that flexible conduit can only be used for final connections (a long time ago there was a rule that flexible cables could only be used that sort of way, but AFAIK it didn't apply to conduit itself, and has long since gone anyway).

There are some limitations though - e.g. flexible metal conduit can't be relied upon as a c.p.c. - so you couldn't for instance use concealed flexible metal conduit for protection against penetration by nails/screws etc under option (ii) of 522.6.204 for example - but that need not stop you using flexible conduit for a bit of extra protection or convenience of future re-wiring if you've satisfied 522.6 in some other way (e.g. runs in zones + 30mA RCD).

In some places (e.g.. France), singles in flexible plastic conduit, embedded in walls etc is the perfectly normal way of doing things in domestics - much simpler to install & draw into than rigid when you have lots of structural obstructions to work around, and makes future modifications a lot cleaner & simpler.

  - Andy.

I'm assuming that as you ,ention the zones this is plastic flexible conduit, and not corrugated earthed metallic. This is unusual in the UK, but certainly  not prohibited by regs . Can I ask  what sort of cables are you putting in? - as a general rule you can get rather less far in the corrugated stuff before it jams, compared to the smooth wall straight stuff.
Also sometimes used more as you would capping, i.e with cables that have an outer sheath, although singles would also be permitted so long as there are no gaps in the coverage.

Mike.