1. I have been told that it is a requirement to have a room thermostat on
the heating system even if every radiator has a thermostatic valve. Is
this
the case? It would seem to be a nonsense.

The requirement is for a "boiler interlock". This means that the boiler
should entirely shut down when there is no call for heat. It isn't allowed
to keep the circulating water hot. This is what would happen if you have
TRVs on every rad and no signal to tell the boiler not to bother.

In 95% of cases, the requirement means that systems are designed to have one
room without a TRV, with a room thermostat in this room. The room thermostat
turns off the boiler completely, which is much more energy efficient.
However, there is another solution suggested in the regs, which is to have a
flow switch on the circuit. This detects when the TRVs are largely closed
and turns the boiler off.

The regs are not prescriptive in any sense. Provided your solution prevents
any gas being burnt at all when the rooms are up to temperature, then it
would be acceptable. The room stat and flow switch solutions are only
suggestions as to how to achieve this.

2. Because of the proposed ground floor location of our new hot water
cylinder our Aga will not be able to heat it on a gravity thermosiphon.

Speak to Aga. It may be that a reliable gravity circulation is required for
safety reasons. This is commonly the case with solid fuel appliances. On
gas/oil, secondary safety systems (i.e. manual reset overheat protection)
may allow fully pumped operation. However, I'm not an expert on Agas, so
don't know what systems they have installed.

Christian.