Yes Hounslow is too dense it is permit parking as well so use the carpark there, but if you want to park for free go to Egham which is much more rural and very close to T5, there is no residents parking and you can in fact easily find a place to park that is not even in front of someone's house so you won't be annoying anyone. Royal Holloway is closed for summer break, so the streets are empty (its harder to find parking during term as many students just park on the street).

I was looking at parking in LHR in a few weeks, but it was ridiculous at like 150 GBP for 5 days so for me cheaper to uber there and back, but for people who live far, that is not an option.