Well, tonight I had my first cheesesteak sub that actually resembled a cheesesteak since I left the US. Mind you, it couldn't touch a proper Philly cheesesteak, but at least it was close.
You can find Steak 'Um like sandwich steaks at Asda (Asda's own brand) and provolone at Sainsburys. Most grocery stores now stock some sort of sub roll. So the above combined with some fried onions and hot peppers made for a half way decent sub.