Carrying CB90 escorts in an amphibious well deck

Could a British amphibious ship carry modern motor-gunboat equivalents to escort its landing craft?

Yes; sacrificing one LCU position could make room for two, perhaps three, armed CB90 escorts.

A CB90 would be a more suitable landing-craft escort than a rigid raider fitted with machine guns. In an Albion- or Bulwark-type well deck, removing one LCU might create space for two and perhaps three CB90s, depending on stowage. Those boats could accompany landing craft through the exposed approach to shore and provide a dedicated fast escort. The trade is less heavy lift in exchange for protection and tactical flexibility close to the beach.