Based on three 2 week trips in the mid-90s, a 2 month trip in 1997, a 2 week trip in 2000, another 2 month stay in 2000 and a weekend trip in 2002.
One of my trips was an exchange program with a boarding school near London (Hockerill School). Most of the others were volunteer stays combined with some traveling. In total, I worked 5 weeks with the British Trust for Conservation Volunteers (BTCV), 2 weeks with the Royal Society for the Protection of Birds (RSPB) and one week with the National Trust. All of the conservation work was in beautiful areas such as the Norfolk and Suffolk coast, the North York moors, the Lake District and the Orkney Islands. I also took part in Ecotopia 1997 near Edinburgh (Ecotopia is the dream village organised by EYFA every summer). When I was at university, I did a 2 month biology internship in Norwich. And of course, on all of my trips, I always spent a few days in London.
England has some beautiful national parks, particularly in the north. However, to really experience some wilderness, head up to the Scottish highlands. The Scots are also great fun and very hospitable.
Great cities in the UK are of course London and Edinburgh. But there are also many nice smaller towns such as Oxford or Cambridge, Stratford-upon-Avon and Scarborough, to name just a few.
I am planning on a weekend trip to London soon some time soon and will collect a couple of useful suggestions and links while I am there...