Do modern toilets still accept a low pressure supply?

The valves are the same. When using high pressure (i.e. mains) a flow
restrictor is inserted into the valve. This normally consists of a threaded
plastic insert that you screw into the water passage.

I believe that all modern toilets have a 6 litre maximum capacity (hence
the
rather pathetic looking -IMHO -diminutive cisterns). Is this correct?

Modern toilet pans are designed for the lower water flow. They work just
fine (and I'm quite capable of testing them to the limit). Don't fit a 6
litre cistern on an old pan, though, unless you do some testing first, as
they might not be reliable.

Christian.