new hybrid commuter bike for someone who weighs 135kg

^^ what @Howard said

That's broadly correct, but I wouldn't get too hung up on strength and durability. As people have said, a bike at your price point from any reputable brand should stand up to the punishment for a decent amount of time.

cx bikes were orginally just road bikes with knobbly tyres. Now they're a specific type of bike that looks like a road bike, has racey geometry and bigger tyre-frame clearances (and often cantilever brakes) so they don't get clogged with mud. They're not really like a mountain bike, although they arguably have similar capabilities to some rigid mountain bikes.

"a light mountain bike with thinner road tyres." would be a hybrid. You can get hybrids with 26" (MTB) or 700c/29" (road) size wheels. There's nothing wrong with hybrids and one would serve you well, but aren't brilliant at any one thing. Some people on here argue that hybrids are a bad choice because many people end up "graduating" up to something more specifically suited to their needs.

Tourers used to be built in a similar way to road racing bikes when everything was made of steel, but with heavier tubing and geometry more suited to stability when loaded up with panniers etc. They also have all the extra eyelets and fittings required for pannier racks and mudguards. As racing bikes shifted to aluminium and carbon fibre, the classic steel tourer was maintained as a design because steel can easily be made into a comfortable frame for eating up some miles and because shaving weight off was less important. Also, for the more hardcore tourer, if your frame broke mid-tour the local bloke with a welder could patch it up; not so with aluminium or carbon. Tourers aren't sexy so you can get second-hand ones cheap. You could buy this Dawes Sterling, upgrade it with V-brakes and matched levers, bar-end shifters and still have change from £300. (next stage would be to buy a Brooks saddle, hang a Carradice off it, start wearing leather cycling shoes, grow a beard etc.)