Easy thing to do, go back to the old days of ERT exclusive to hotel guests and annual pass holders only. Instead of the free-for-all it is now. ERT has devalued itself since the day it was available to everyone.
Probably on a more realistic outlook, an easier way to manage it could be to draw a line between who can and cannot get ERT. Those who use offers from 'external' persons, like the Kellog's 2for1 offer cannot get ERT, those who buy via Alton Towers 'without' offers get ERT. I know both ways you buy via AT.com, however Towers can easily word it to ensure the general public don't catch on to that.
OR they could simply let online bookings run on half price offers and offer ERT at £X extra per person. So if you want ERT you pay more in effect. Keeps it as a 'perk' and everyone's happy.
So you have:
- Parking Ticket: £5 (£1 discount)
- Theme Park Ticket - With 2for1 Code: £24pp Without ERT - ERT Offered at £15pp - £39pp with ERT
- Without Code: £36pp Exclusive ATR Offer inc. ERT
- On-the-day: £48
I think that would balance out things nicely and also not damage the perk of ERT (for a family of 4 using 2for1s they would avoid the £60 extra just for the sake of ERT). The majority of people use 2for1s on park these days (not sure how online sales do) so I doubt an online move would severely affect Towers. Baby steps are the way to go. A drastic move would be silly. But slowly moving people towards buying tickets online would be a good start.
They could start doing online parking tickets now really. Just have something that you can print at home and show the man at the barrier when you leave the park!