Theatre is a mobile profession, and transfers between one theatre and another are commonplace. They are also — usually — a sign of optimism on the part of the producers.
Many productions tour before they come into the West End (Bath, Windsor and Richmond are three particular favorites with producers who want to break in a show before London), but these count as a tour rather than a transfer. A transfer is when a show moves from a fringe/off West End theatre into the West End, or from one West End (or equivalent) theatre to another, or a subsidized theatre to a commercial West End playhouse. The other category — the ultimate in transfers — is from London to New York.
When a play does well in a fringe theatre, its producers can sometimes raise the money on the back of good reviews and profitable box office and transfer into the West End — a recent example was The Glee Club, which moved from the Bush to the Duchess.
The next stage up from a fringe theatre is an off-West End one, some of which are — in terms of artistic quality of the productions and caliber of actors they can attract — West End in everything but geography: Hampstead Theatre is one of these, as is the Almeida. Both theatres have often sent productions into town. Hampstead sent a memorable version of Joe Orton's What the Butler Saw to Wyndhams, and the Almeida skipped a step by booking the Albery Theatre for an "Almeida in the West End" season that included Dame Diana Rigg in Phedre. The risk with these sorts of transfers is that the "natural" audience for the show will have already been to see the production at its original venue. If there hasn't been a high-profile cast change, then it's unlikely that the play will be re-reviewed, so in publicity terms the transfer can be something of a flop, however nice it is for writer and cast to see their names in lights on Shaftesbury Avenue.
The King's Head, Islington has had more transfers to the West End than most, and one such example was the Sheridan Morley-directed last play of Noël Coward, Song at Twilight. One of two female lead roles was played in Islington by Nyree Dawn Porter, but for the show to stand a chance in the West End (and, crucially, to get new reviews) she was replaced by a bigger name, Vanessa Redgrave.
The Royal Shakespeare Company, when its London base was at the Aldwych, was in the business of transfers from its base at Stratford, but these weren't transfers in the general sense of the word, as the London showing was pre-planned before each play even opened at Stratford. The National Theatre, on the other hand, has both pre-planned transfers (basically the big musicals like Oklahoma! and My Fair Lady, whose development by the National had been assisted by producer Sir Cameron Mackintosh) and conventional ones, where the success of a play at the National prompts it to move it into the West End — as with Blue/Orange at the Duchess and Humble Boy at the Gielgud for example.
To then move to the next, vast step across the Atlantic happens very rarely, though in the National's case, Noises Off did just that, as did Oklahoma! (from the NT to the Lyceum to New York), while a current example of a "commercial" (as opposed to subsidized) production crossing the Atlantic is Duncan Weldon's Private Lives, both of whose stars (Alan Rickman and Lindsay Duncan) are up for Tony Awards.
A theatre that lies somewhere between the subsidized and the commercial is the Donmar Warehouse, which has also had transfer triumphs, though in its case they are always direct from the Donmar to New York, as in The Blue Room and Cabaret, for example.
Transfers from one major London theatre to another can work: Noises Off moving from the Piccadilly to the Comedy, Art moving from Wyndham's to the Whitehall, The Mousetrap from the Ambassadors to the St Martin's back in the 1970's, but others can end in tears, like Mother Clap's Molly House which, when it moved from the National to the Aldwych, was unable to generate the audience that it needed.
So transfers are not automatically a good thing for a play, but in general they are a sign that it's working, that its producers have confidence in it, and that the omens are good. Whatever the end result, a transfer to the West End (or on to Broadway) is many people's dream, and a sign of the versatility and cross-fertilization between fringe, subsidized theatres and the theatre stock of the commercial West End.
—By Paul Webb Theatrenow