Just a heads up that the rowing regatta takes place at Henley-on-Thames during the first week of July.
From London (Paddington) take a stopping train to Twyford where you change for the branch line to Henley. No need to advance book - just buy the return train ticket on the day if coming from London or Reading or anywhere in between. Expect loads of people!
You don’t have to pay to go in but you will see various enclosures / hospitality for various clubs/ colleges/ corporate. You have the legal right to walk along the Thames Path - which is a bit narrow with all the crowds - as it is a public footpath.
From the station, you might want to wander around the town first before heading down to the river bridge. Cross to the other side and then turn left.
https://www.hrr.co.uk