I suspected that eBay is a better place than a car boot sale to try to sell an issue from Jan 31 1987, so had a quick look. They go for about £5 for a job lot of ten issues, so that#s 50p each, which is 32p more than they cost at the time. However, since 18p from 1987 is worth about 53p today, according to the Bank of England's inflation calculator, they're not a good investment.
I don't know how much the owner wanted for his comics (he was selling the entire crateload as a single item), because I didn't look. I have better things to do with my time than sorting them into groups of ten to flog to collectors. Also, my wife was with me, and her primary reason for going to care boot sales is to buy nothing.