Michael
Cars registered on or after 1st August 1998 do fall under stricter rules when it comes to fitting towbars.
If your car is registered on or after 1st August 1998 and you fit a towbar, it MUST by law be approved to EC94/20 and installed correctly according to the manufacturers' instructions. It is an offence under the vehicle Construction & Use Regulations to fit, use or permit to be used, a non-approved towbar on the vehicle - see https://www.towsure.com/static/towbars_ec_type_approval
I am not aware of any EC type approved towbars for the LS400. The Watling towbar for the LS400 does not have type approval. That is why it states that for UF20 cars it is for those registered between Jan 1995 and Jul 1998. It is not that it will not fit Aug 1998 onwards cars, because it will, but it cannot lawfully be fitted as it is not a type approved towbar.
I am sure there are Aug 1998 onwards LS400s fitted with a non-type approved towbar. If one was fitted, I do not think you will have any problem at MOT time but you might have in the event of an accident if your insurance company became aware of this fact.
It might be worth looking into whether there are any EC type approved towbars for the LS400 as it would certainly be better to fit one if you could. I am not aware of any but that is not to say that there are none. There may be one out there somewhere.