my personal view is that they are mine rather than my company's (leaving aside the actual legal definition which I understand). A friend and colleague of mine once put it that unless he only travels between 9am and 5pm (which never happens) and/or the company give him time off in lieu etc, he's blimming well 'entitled' (probably too strong a word) to them.

interestingly, different legal discusisons in Germany triggered a few years ago by MPs using miles (and status) earned on official travels for personal holidays.

Suspect there could also be some 'benefit in kind' potential questions around use of miles for personal travel accrued from flights paid for by your company but suspect it's too small an area / too messy for HMRC to be really bothered about it.

I have heard of companies though (and am sure those with a much more vested interest in the company's bottom line (i.e. typcially not large corproates)) where directors, individuals, owners use accrued miles for business travel but I don't think that would even by possible at my company given we have to go through a corporate TA and I am not sure I could use Avios and then claim tax and fees on expenses.