UKH

I’m old and crap at bouldering so fall off a lot but wish to improve😀 please recommend a suitable mat at a reasonable price!

Define reasonable? 

Affordable for an oap😀

I extensively researched around a year ago. 
Snap and Ocun offered specs that interested me but there were some bits of innovation I possibly wouldn’t have paid for given the choice.  I would be tempted to check for deals across all brands; unless you want some specific safety feature.  A friend and I looked at a fair few specs before we came across the mistaken price we eventually found and bought two. 
I’m not being helpful I know but Ocun stuff feels good generally. 

Moon probably have one of the best carrying systems, I'd go for one of them or an Ocun Dominator. Thicker pads tend to have an additional soft layer of padding on top of a standard pads firm layer, so that's likely a key spec to look at. 

Alpkit will rent you one for £10/day, so you could do that a couple if times, then buy if u like it. Think not that pricy, AND made in UK, which is probably not common. Good luck.

Snap pads are notably better than simple foam ones at cushioning falls, I don't regret getting my Grand Wham. Not cheap though....smaller ones are available mind. 

I’ve been very impressed by my Moon Warrior - quite big, well made, practical carry system - I see RocknRun are doing them for £198 which isn’t bad.

Even though, I don't have old knees (well, only middle aged). But I do have bad knees, one of which has been operated and reconstructed twice now...

I'm still un-sure, what you want "feature" vise from the pads?

Either they are stiff and supportive, so they make a nice landing platform... or they are soft and plushy, making a forgiving landing zone. The former will give you a "hard" landing that can be bad for the knees... and the latter will give a nice soft landing, but can also cause twisting on the knees that can be also bad...

And if it not stiff, nor soft soft, you might get something that will be hard and yet prone to bending/twisting...

I'm gonna go out on a limb, that perhaps the stiff/supportive is not what you're after (ie. hard fall). Which pretty much means that you should look at Snap pads, the ones with those aircells in them (like the Wham, or Grand Wham mentioned above). Due to the construction, if you fall from small heights, they feel nice, soft and plush. But the higher you go and fall, the more supportive they became (a bit like D3O impact protection... soft until there is a high impulse/energy hit on them).

I do like my old P'tit Wraps (now discontinued model, but I'm sure something like that still exists). And I've used a them a lot, and they still are in fine working order...

+1 for the Snap Grand Wham being very forgiving to land on.

Yes. I can confirm the Grand Wham and also the Ocun version of the air displacement landing (Paddy Generator or something equally unmemorable.) work really well. They aren’t cheap though, unlike this Metolius https://www.alpinetrek.co.uk/metolius-bergfreunde-magnum-crash-pad/