The creation, gestation, birth and production cost of a new musical is an order of magnitude more difficult than the similar process for a piece of drama, and in the UK (unlike in the USA) there is no 'greenhouse' to nurture the creators of musical theatre.
This is not to say that the creation of a play is easy, for it too has its difficulties, but the large musical, with its crucial inter-dependence between book and score, its larger team of interpreters (director, musical director, choreographer etc), often larger cast size and its generally larger production scale, conspire to throw up problems not usually faced by the producer of a straight play.
We strongly believe in the need to develop and support the talents of British writers and composers and consequently we have a strategy of representing 'homegrown' creative people wherever possible. So far we've contracted these entirely British (mostly full length) musicals ...
(** means writer/composer is a member of Mercury Musical Development)