@Yoffel
HO WC is Horizonal outlet. So the soil pipe goes out of the back (usually, sometimes sideways) of the pan, through the wall or into a duct. This is most common current UK practice. So there are a great many to choose from, including some "short projection" pans suitable for cramped situations.
BO WC is bottom outlet.
Not what you think, it means that the soil pipe emeges downwards from the trap and goes into a socket in the floor (usually a solid concrete, ground floor).
It is a lot of effort to dig up a concrete floor and move an old clay soil pipe.
So the pan design must have the pipe the same distance from the wall (ish) as the original. In this case IIRC it was about 10 inches/250mm. There are not many BO pans made by reputable British makers now. Armitage make some traditional styles popular in restoration of older houses and hotels.
Makers who are not reputable produce WCs with a poor and ineffective flush. You may have heard people grumbling about their modern toilet.
An air vent into the next room is very unsatisfactory. The occupants of that room do not wisg to detect by their senses the use of the WC.
You can get very quiet modern extractors now. 25db or less, with an airflow of 80 cu.m/hr or more. They can be wired to the ceiling light so they come on when the light is turned on, and run on for an adjustable period afterwards.
You can get an amazingly cheap one that seems to me pretty good
or a really good one with a ball bearing motor that is quieter, more powerful, uses less electricity, and has an integrated backdraught shutter. This is my favourite. The Envirovent 100 T seems identical and is cheaper.
There are some other brands popular with builders because they are very cheap. Even some well-known brands are not as good as the ones I show above. I have been particularly unimpressed with Manrose.
if you buy some other fan, look for noise rating around 25db, and nominal air throughput of 80 cu.m/hr or more
A bathroom with steamy shower needs a more powerful fan.
i also recommend fitting a solid-core or fire door to the WC. These are very solid and dense and block out noise.