I would not wear shorts above the knee or a vest top in Indonesia. You will get stares and feel out of place. We saw some clueless young western women in super-tight shorts and tank tops getting very hard stares in Kuala Lumpur, and Malaysia is considerably more relaxed than Indonesia, so you need to allow a margin before it gets to that point. No shoulders, no thighs on show. Apart from anything else you don't want your shoulders getting direct sunlight from overhead. T-shirts, and baggy shorts no more than an inch above the knee, will be fine.
For transfers and any sort of road journey, install two apps, Grab (which also works in Malaysia) and Gojek. We had both and could probably have done with only one, as they always both seemed to have coverage, so sometimes it was eeny-meeny to decide which to use. At some major terminuses there will be Grab and Gojek reps in hi-vis uniforms, which we were wary of but they are just there to help you use the app if you are unfamiliar. Do make sure you have registered a payment card and made an all-but-final-confirm booking on each one before you rely on it to get you from the airport. Both apps are extremely cheap. A one-hour ride from downtown Jakarta to the airport with Gojek was about £6.
There are about 20,000 rupiah (Rp) to £1, so when the hotel rings up the bill, check there isn't an extra zero. We never encountered anything that remotely resembled a ripoff, but it would be easy to get wrong by accident. Sometimes you will see prices with the last three zeroes missing, at which point the abbreviation changes from Rp to Rb.
Do not hesitate to eat satay and other freshly-cooked street food. It's very cheap and very good. We took loperamide but didn't need one tablet in nearly three weeks of eating in local places in Malaysia, Indonesia, and Brunei. On the other hand, do not drink the water, don't even brush your teeth with it, and refuse ice unless you are satisfied that it's made with bottled water. Hotels will usually provide bottled drinking water, either refilled from a filter machine or in sealed plastic bottles.
Most food is spicy, but it's usually not blow-your-head-off level. If you enjoy a curry you will have no problems.
Food is incredibly cheap. You can get a bowl of chicken and rice for £1 or so. Soft drinks are also cheap, but not to the same extent. A Coke might cost half the price of the chicken and rice dish.
It is hard to find alcohol outside of hotels that cater to westerners. It is expensive, and wine is generally terrible. Curiously, the price premium for cocktails over, say, a bottle of beer is not as great as you might think - you can get a reasonable cocktail for £10–12 in most 4/5-star hotels.
One small culture shock for us was that everyone smokes, all the time. Cigarettes are extremely cheap.
The level of English is not especially high, but Bahasa Indonesia is written with the Latin alphabet, so at least you can type words into a translator app (I recommend DeepL).
If you have not already got your visa-on-arrival, it's not too late to do so at https://evisa.imigrasi.go.id/. Our e-visa was issued within 30 seconds of the payment going through. Then, a couple of days before you travel, get your e-landing card at https://allindonesia.imigrasi.go.id/ (remember that your arrival date will typically be the day after your plane leaves the UK). With those two done, you can walk straight to an e-gate and insert your passport - no need to see an officer and you won't get a stamp. If you don't do either or both of those then don't worry, the "You must fill out the landing card before boarding" warnings you might see are not absolute requirements and you won't miss your plane, but you will have to faff about with them on landing while jetlagged, so it's really worth doing it before you set off. All that then remains is customs, where they X-ray every suitcase while you stand next to a cheery sign reminding you of the death penalty for drug smuggling.
You will have an amazing time. The people are wonderful. We didn't feel out of place or uncomfortable at any time. The country is not rich, but nobody is starving and you will not feel like you are "a target" for being western.