3NT denied a 4 card major
2NT was 20-22 points, the "standard" range in the UK. After East has denied a 4 card major, East-West have no agreed suit, so West can make a natural try for a slam in no-trumps by bidding 4NT. This shows about 11 points (in a flat hand like this) and asks East to bid the slam with a maximum. 21 points might be enough for slam if East had a 5 card suit, but with a completely flat hand, East passed. 11 tricks are the limit in No-trumps unless South leads the Ace of Diamonds.
Six Clubs can be made, but bidding it is another matter on standard methods. With a slightly better hand, East might try bidding Five no-trumps "pick a slam" but in West's shoes I would not then bid a 4 card suit headed by the 10 at slam level.