Ian,
What tyre are you going to fit to you P4SS? I’m in the process of fitting Minilites to my Plus 4 and want to get tyres that provide good wet and dry grip for spirited driving.
Roger
As Kevfitz said I fitted Toyo Proxes. I stuck with the original size at 205/65 R15s and got the Toyo Proxes CF2 94V. You can see the spec and wet grip/noise ratings etc. on their web site
https://www.toyo.co.uk/tire/pattern/proxes-cf2.I've only had them for a few cold weeks and under the current restrictions haven't done many miles. Although having done a fair bit of tyre testing at an OEM during the earlier parts of my career I can say that I have no real negatives with the tyres and within the constraints of a Morgan chassis they perfrom well.
I think they are often specified for light commercials, as when I ordered them from my local tyre depot he asked me what van are they for. .

That probably explains the XL availability although I confirm mine are not XL.
Not sure if you need XL for such a low mass car as a Morgan although they might provide more side wall stiffness which could give better steering response, especially at small steering inputs.
Certainly the non-XLs had lots more grip and feedback than the overaged Avons they replaced. But no surprises there.
They are also very comfortable taking out a lot of the small sharp impacts. There will probably be a difference in the XL and non-XL in that respect.