Unusually I feel sorry for the LBS.
If I were eligible and dug my five year old Argos (not the people in Bristol) special out of the garage and took its to the LBS what are they going to do?
1) Look at it and say sorry it will probably cost us more than £50 without even doing an inspection. After which the shop will have a reputation for being run by stuck up <word of your choice> who wouldn't even look at my bike because it didn't cost over £5k.
2) Look at it, do a free thorough inspection and say sorry it will cost us more than £50 at which point the customer says I can be a new bike for less than what you say it will cost to fix. So the shop has wasted say half an hour of the employees time and are rip off merchants.
3) Look at it, say a thorough inspection will cost £25 regardless, leaving zilch left in the voucher for actually doing the fix, at which point the customer says I though the inspection was free and the shops reputation will be affected in the same way as 1)
4) Even if they say that the aren't in the voucher scheme I expect some will still call them names and expect a free checkup.
So the LBS loses if it joins the scheme and it loses if it doesn't.
Given that many tyres don't last more than 4-5 years, if you are a retailer with the liability for passing them as safe, rather than an individual trying to make them last a bit longer, aren't most bikes brought in under this scheme going to need 2 new tyres, which need to be fitted possibly with new tubes? That's £50 in parts and labour already.