The most open totally man-made water I can think of is Carsington Water in Derbyshire. I think I read somewhere that it was the most popular tourist attraction in the whole of Derbyshire, which, given the Peak Districty nature of that county, is quite something. I compare it to the large reservoirs in Essex which are hardly used by the public at all by comparison. Abberton has some scant coarse fishing and an Essex Wildlife Trust cafe, Hanningfield likewise, but a bit more so.
I don't think there's a great deal of difference between the Essex and Derbys. reservoirs other than possibly, the steepness (in places) of the concrete bank. I do recall that the dam at Abberton was taken off-limits for anglers after some nefarious activities in the 1970s (I think) which was at the time attributed to the IRA. But I would have thought that that particular threat had receded somewhat. That dam was also the site of the biggest haul of pike in one weekend that I am aware of - a small group of blokes caught 721lb of pike over the weekend, and the fish averaged 19lb apiece (if you can get hold of it, there was a detailed report in Angling Magazine in October 1970).
