Yes, you'd be very unreasonable to withdraw your child from food tech lessons, and I'm not sure the school would be fine and dandy with that, anyway.
It's pretty hard to make edible ingredients actually become inedible in the time available to school kids in a food tech lesson, unless there's either basic food safety failings at play, or purposeful sabotaging of the cooking is going on.
You say he's not a bad cook. What do you think is going on to make the foods you sent him in with inedible?
But, £5-8 quid every lesson to get kids to a basic level of proficiency in food tech shouldn't be necessary, so perhaps it's worth a discussion with the school as to their approach. For comparison, most of things that are made at my kid's school use in the main things that people who have kitchens and use them to prepare meals are likely to have anyway (i.e. 'store-cupboard staples'), and a small amount of fresh ingredients. I could get to £5-£8 each time if I had to buy the minimum available quantity of each ingredient from a supermarket each time (e.g. for an omelette: 6 eggs, bunch of spring onions, block of butter, block of cheddar, one red pepper), but given the actual quantities used tend to be a fraction of what you can buy, the ingredients for lessons at my son's school usually work out at less than £2.50 a time, and it usually produces something the kids can eat for lunch. If your child's school isn't doing likewise, perhaps that's something to bring up with them.