This one starts at the famous Henley bridge and heads 14km to Harlow. It’s strictly for challenge-seekers, but does come with the added incentive that the swim is downstream, which makes it much, much easier. 

£77, Aug 11, 6.30am 
Henley-on-Thames, RG9 2LW 
Station | Henley  
henleyswim.com

Photo: Getty