Hello Eddie,
Welcome to the Forum.
There are many conversion companies.
As OldWellhouse says, these three you mention are all well known and reliable.
Unfortunately they are not close to each other. Calder is closest, I think.
NorthStar is IOW !
Are you expecting to buy ready converted ? Side or rear conversion. Pop Top ?
The first questions you need to ask is "for what purpose(s) am I going to use it ?"
How many people will be with me ? Dog(s) ?
Will I cook in it, gas or induction ?
Do I need a fridge ?
If I will sleep in it, for how many consecutive nights, etc.
Will I have an awning or a supplementary tent; or neither ?
The Club is organising a Meet in Warwickshire in Autumn.
You would learn a lot there and be able to immense variety.
Good Camparding