I think you've got it more or less right Malc. The £29.50 includes £5.50-worth of "free" calls and as the charge for calling the UK is 50p/minute that means the basic card costs you £24 and you get 11 minutes' worth of calls before you start paying extra.
My PAYG on Orange charges me 90p a minute to call the UK from Greece, so I'd get over 32 minutes of calls for £29.50, as opposed to the 11 minutes you get on the SIM card. But if I made 32 minutes' worth of calls using the SIM card it would cost £29.50 + 21 minutes at 50p = £40. So you obviously need to think about how many calls you're likely to make in order to decide whether it's a good deal for you or not.
If anyone needs any help in working out what the break-even point is, let me know what your current charge is and I'll work it out for you.