1 Hadrian's Wall
Even the Emperor Hadrian wanted to keep a barrier between his empire and the "barbarians" of England's north. He took six years to build his wall, begun in AD122, and although farmers over the millenia have since helped themselves to its stones for their own wall projects, there is still plenty left to explore - although climbing on the wall is not encouraged. The original wall was about 80 Roman miles long (117.5km) and made from whatever materials were to hand, including turf and wood. Today, Hadrian's Wall is a World Heritage Site.
2 Stonehenge
Well, if you're going to visit ye olde England, you can't get much older than Stonehenge. Actually, it qualifies as ye prehistoric England, because that is what it is. Exactly what it is - a coronation place for Danish kings? a druid temple? an astronomical computer? - is still open to much debate but it was immensely important to its Neolithic builders, who spent about 2000 years lugging great stones hundreds of kilometres and putting them together.
3 Magna Carta