Like has been said on here, many phone packages incorporate free calls. These 08 type numbers are pretty much always exempt from these packages because the person you are phoning is getting a kickback on your call. It makes me laugh companies who use these 0870 number on their sales and support lines, personally I will go out of my way to avoid such companies.
So, you call your surgery, and then find out that the doc you want to see can't see you for x reason. Also then you can't future book because they don't accept appointments in the future, this I think was a method by which the 'GP waiting lists' were 'Enron-ed'.
How about another tack, If you make an appointment with your GP, you fail to turn up then you get a bill for the wasted time, maybe around £10-£20. This then goes against the 'health' account of your address, which means that no further appointments can be booked for that address until the account is paid. Failure to do so will result in same recovery actions as is possible with the great value council tax, deduction from salary, benefits etc.