£100 can buy you a great bike second hand. Many people spend £200+ on a half decent bike, ride it a few times and then sell it for £100. I've seen loads of nearly new bikes on
ebay from Giant, Trek, Specialized for £100. They are mostly the Hybrid type bikes with 700c wheels and low end Shimano components, but still far superior to the stuff knocked out by Asda etc..
Many of these BSOs appear for sale as job lots on ebay because they have been returned to the retailer as faulty and passed to wholesalers to dispose of.
This week I saw a super 1990s Raleigh Elan (one owner from new) Reynolds 501 lugged frame sell for £60
1980s Raleigh Clubman 531 tubes and forks £60
Proper bikes for no money