There are two ways to do this. The cowardly way would be to find a lonely bend in the Thames, get in the water with shorts on and remove them under water, carefully watching to make sure there's no one around, and trying not to attract attention.
I, however, will buy a ticket for a family swimming session at the Windrush Leisure Centre, and strip off in the changing room. I will march proudly to the pool, shout a cheery 'Good morning, families' before entering the pool with my famous 'sea lion' dive. People like confident people, so I expect quite a lot of the families and staff will come up and congratulate me on my boldness.