I would prefer not to use National Express. Conditions on coaches can be awful, in my experience. I wish we had something like Polskibus in the UK.
Are there any alternatives? I mainly use them to get between airports and Birmingham when it's dark.
There are a few other companies in the UK, although none are particularly great - it's not huge distances so they go for packing you in, more than anything.
Bonus - EasyBus - although only in London at present, if you get it in advance, tickets can be very cheap to the airports, from as little as two pounds.
Another altenative maybe to consider the train, if of course you can find a cheap ticket or one that is similar to the price you would pay on either National Express or the other companies mentioned in Mark's answer.
Cheap train tickets is a resource zone for finding cheaper train tickets. It's packed with hidden tips and tricks for cheap rail fares.
We’re a partnership of transport companies, local authorities and passenger groups which have come together to bring you routes and times for all travel in Great Britain by bus, rail, coach and ferry.
To get the coach services only deselect everything but
coach in the travel mode settings:
This will show in the search results who the particular route is serviced by, for example, the route from Tiverton to London (Hammersmith) is serviced by Berry's Coaches:
You can then search for that name on the Internet and see who runs it and on what other routes.
Here's one more, serving the Birmingham area which you seem to be interested in:
Thandi Express runs daily scheduled services between the following towns - Wolverhampton, Birmingham, Coventry, Luton, Barking/Illford, East Ham/Stratford/Manor Park (East London), Woolwich, Charlton, Dartford, Gravesend/Rochester/Strood/Gillingham (Kent) , Southall, Slough (West London), Leicester, Nottingham/Derby, Sheffield, Bradford/Leeds/Huddersfield/Dewsbury.