We are up against a legal xenomorph whose bad ideas hatch in people's brains!
Yes, the rules would be different.
If it were called "Space Marines ®"
If it's "Space Marines™" or just "Space Marines", it will just be "Fooock Ouuufff!".
A quick look.see at the USPTO (is there something like that for the UK)?
"SPACE MARINE" registered for "video computer games; computer software for playing games"
"SPACE MARINE" registered for " board games, parlor games, war games, hobby games, toy models and miniatures of buildings, scenery, figures, automobiles, vehicles, planes, trains and card games and paint, sold therewith."
both registered to
"GAMES WORKSHOP LIMITED CORPORATION UNITED KINGDOM Willow Road, Lenton Eastwood Nottingham NG7 2W5 UNITED KINGDOM"
as a TRADEMARK