Hiya,
Swimming at Haversham and Brogborough lakes, confirmation should be available on the TeamMK forum/website in the near furture. Im not a member of TeamMK and do my OW swimming at Datchet (near Heathrow), just a habit I guess and a popular venue with other Pirates, oh and they do a nice sausage and egg bap.
Lots of quiet lanes around MK but id not cycle on any of the grid roads or the A5. The A5 south of MK is a busy fast road (60/70mph), I couldnt think of worse places to cycle but ive seen some cyclists using it.
There is a large cycleway system in MK (Redways), theyre safe and ideal for commuting but not really suitable for roadbike speeds, better suited to mountain bikes.