If you're lucky you can find a 10-20 year old, name brand, rigid mountain bike in like-new condition for about $100 in the USA. Might or might not find similar deals on the used market in the U.K. Extremely similar to an "urban" bike and far cheaper. Those bikes are neither light nor particularly heavy, but they tend to have very durable wheels and frames- particularly if you're willing to tension and true the wheels by hand. But it's a lot harder if you're looking for a bike for someone who's 6'3" than if you're looking for a bike for someone in the 5'6" to 5'10" range