Do Alton Towers need to use a BOGOF promotion, no, they have 1.3 million likes on Facebook, which is just under half the number of visitors they had last year (2.7 Million I believe).
Combine this with a sensible TV/Radio/Cinema campaign through out the year, I think you can reach just as many people as the BOGOF does.
As for vouchers been linked to customers getting a good deal, that's fair enough, but it is also linked to a cheaper product, something that is not a premium product, and I am a big believer that the experience at the park should be something that leaves customers wanting to come back year on year, and not needing a push from a advert campaign. If people want to come back, they will tell their friends and family.
As for the price of tickets, and how they are sold, I am not a fan of selling adult tickets to 13 and overs. Adults are 18 and over, and if you want to charge children tickets and adult tickets, you need to use the "Legal" age limits, otherwise it leaves a bitter taste in peoples mouths.
I think a fairer way of charging is under 0.9m is free, under 1.4m is £10 and 1.4m or over is £30. This way your only charging people for what they can ride on, and you don't get in to that silly age row. You also won't get people complaining that they are being charged full price, yet their child can not go on any of the bigger rides. (These prices are in the ideal world, where the park is well maintained, and a truly magical day, half these prices, and I feel that would be value for money with the current offering)
As for price rises, this has to happen, and can not be avoided due to inflation. Maybe I would try and limit the price rises to once every two years, so rather than 60p here and there, it was just £1 every 2 years, at the start of season. I think mid-season price rises just appear messy, and like the park is trying to maximise its revenue.
Crofty raised an interesting point about the BOGOF vouchers being a bit of a scam, and I have to agree. They are available all year round, so its not like its limited time, and its a cheap attempt to make a day at the park look more affordable.
Disney don't do BOGOF Vouchers, Europa don't do BOGOF vouchers, and they are the two best parks in Europe, so why Alton Towers (Merlin) feel the need to continue with them, I do not know.