What time is it? Watches and horology

Sharkdiver 40//42//

Now I can't try one on, so what would be comparable in size to try out available in a shop in London. Rolex sub/sea dweller?

Between Stonehedge's girlfriend's 40mm, and an Armida A8 in SW4, you might be able to try both sizes. The A8 is a little slimmer, but otherwise has similar dimensions to the 42 Helson. The Deepsea is 42.999, but 17mm thick. Where are you?