I very much doubt it.
Unless they're offering tickets elsewhere as part of a promotion, such as the cheap tickets available for some London theatres when you travel by rail (
http://www.daysoutguide.co.uk/entertainment-shows-concerts ) all London theatres sell their surplus tickets via the Society of London Theatre's own ticket outlet in Leicester Square. They only offer them on the day of the performance and tickets have to be purchased in person:
http://www.tkts.co.uk/whats-on-sale/
Regional theatres often sell all of their tickets anyway. If not, they'll probably create their own promotions (perhaps, say, in conjunction with a local newspaper) rather than using the [expensive] services of online ticketing agencies.