Not wishing to diss what I'm sure is a well organised race, but you can run round Dorney Lake any time you like for free!! And it's got distance markers up permanently - I use it a lot for threshold sessions. Of course, there's nothing like a race atmosphere to drive you on, but £20 seems a little steep to me - it's not going to require much marshalling, put it that way!
And yes, 4x round the lake will get a bit repetitive - as rowing lakes go, it's very nice though - you might see some interesting wildlife if that floats your boat! :¬)
Elstead - it's a lot harder than Windsor, 4 hills, all pretty challenging but varying in length and steepness. Great fun nonetheless! I've heard it described as the toughest half in southern England, and I wouldn't be betting against that.