I want to know how good or bad is the legal framework for owning,
renting and getting managed appartments in the UK, particularly in
London.

It is pretty good

You can let for fixed time periods and kick them out.

Obviously one can lose if they trash the place, forget paying the rent
for a bit, and disappear - also they may let in squatters - but
legally one is not trapped - provided one gets things right.

This is a reaction to post WWII rental restrictions that made renting
property a mugs game - so it dried up.

In conjunction with your later question, things vary a lot depending
on the 'quality' of the place you intend purchasing to let out.

My understanding is that the top end of the market in London has been
rather soggy for some time - half inch thick, light oak floors are in,
and they don't come cheap.

A good agent will guide you through the pitfalls as the legalities
change depending on whether you rent furnished or unfurnished etc. An
agent will normall take a hefty cut of the monthly income but a good
one will keep a close eye on your property and deal with all the
problems. Expect to cough up at least 15% of your income and do your
research on the agents. Many will guarantee your income regardless of
property being let or not.

Thank you!!