A quick call to customer services prior to your arrival at the M&S store. £30 is the current limit as per t&c's
I think you may referring to contactless payment, for which there is a £30 limit, imposed by the banks as the technology is fundamentally insecure (my words, not theirs.)
There is no limit applied for the bank for Apple Pay as it has that additional security over cards. Of course the retailer may choose to apply any limit he likes, it is a free world. More specifically I hear rumour that the 'generic' limit may be hard coded into some, but not all, of the hardware terminals, meaning the banks' contactless card limit is needlessly applied to Apple Pay transactions too.
All of which is fine, I am not trying to change the world. I just want to know a way to predict, when I get to the til in some random shop, whether or not I'll be able to use my watch for >£30.