I'd be very surprised if you can find tickets for that weekend. 'Travel+theatre' discounted deals are normally only offered for dates when the theatres find it hard to fill their seats (and so offer discounts to coach operators) and travel companies aren't using their coaches for other trips (as they will probably be doing over the Easter period).
Your best bet is probably to look for cheap rail tickets to London (although they can also be hard to come by for holiday periods) and then use the 2-for-1 theatre ticket deals for shows such as Stomp (which I love) or The 39 Steps (which has some great reviews) and then book a cheap hotel.
http://www.nationalrail.co.uk/
http://www.daysoutguide.co.uk/2for1-london
However if any coach trips actually exist when you want them, the Tourist Information Centres in your area should all have details of them (because coach operators normally send copies of their brochures to TICs). Phone numbers here:
http://www.visitliverpool.com/visitor-information/tourist-information-centres