I'll give him some cred for his action work in the Musketeers, by the end of S3 the actors all started looking pretty capable and comfortable with the swords and all the heavy kit, different kind of soldiering to today though :-).
The few SAS I've seen are pretty bulky, I'm 5 11' and saw a soldier shorter than me throw a outboard motor for a zodiac over his shoulder like it was nothing and be on his way.
I mean you have to suspend your disbelief, I like Cpt James cadence in his voice when he gives orders, bit too chummy with the guys but I'll consider it to pass muster. Same with Pasqualino, no doubt he's fit, but I just imagine he's a guy who's got some lanky spidermonkey strength going on.
Face-wise he's not got that hard Bodie and Doyle kind of mush going on like from the Professionals (remember that?) but yeah they obviously want to set the Elvis character apart and make him a viable romantic alternative to her new perfectly seeming fiance.