Flight of Time by Paul Capon

Four teenagers find a time-ship on a lonely stretch of shore. It has just brought two travelers-in-time from the future. When they leave the ship, Clive, Mark, Carol and Jill climb in. The ship starts off and they are carried on through the centuries at breathless speed until it comes to rest in A.D. 2260 and they step out into an unrecognizable England.

First the sea was pushed back to make room for the ever-increasing population, then cultivation was carried out in layers, with fields rising in tiers like giants' skyscrapers. The people are governed by an enormous electronic brain. The inhabitants of this extraordinary world plan to keep the children for ever, as