Mine cost £250 in 2022.
You buy a completed steering wheel from them with an extra £50 (I think) deposit.
Once you swap the steering wheel over you send your old one back to them (in the same packageing) and they give you the deposit back.
The flat bottom is created by the extra “thickening” of the wheel it is subtle but noticeable… and fun 
Royal did the wheel on my car- an ND1 2 litre. I went for a very slightly thicker rim (1 or 2 mm from recollection) and perforated leather on the sides. My car is a Z-Sport with has red stitching which they matched perfectly. Used an exchange wheel so they removed my old one, swapped all of the fittings onto the ‘new’ one. Absolutely superb job, I recommend them highly. Only thing to watch for is the alignment of the wheel when they fit the new one. There aren’t many splines on the wheel/ steering column so it may be impossible to get it completely straight. I solved this by getting the wheel alignment checked (and adjusted) afterwards- which I’d planned to do
