Just buy them in England if needed.

PPs are on point. I'd add that if you find you do need them, you can buy them there and you'll get a better selection. Ditto other rain gear.

You will be fine with ankle height boots in England but for a few months though it's also a personal preference to some extant. You have to adjust your wardrobe for either. Think about what you'll be doing and how you'll feel if your lower legs are a bit damp after a commute. This doesn't bother me but I default to tights with skirts or dresses or slim fitting pants in materials that dry easily (and cropped enough that they are never totally soaked except in a deluge).

Actually my go-to is a pair of waterproof Chelsea boots that come a little less than halfway up my calf. I think they look more professional and cuter than any wellie option and I protect my legs well enough without being as full coverage as a traditional wellie. The issue with the ankle-height ones to me is that they look like gardening shoes (in addition to offering less coverage).

Also consider you rain jacket situation which is arguably more important than the boots. I recommend something at least mid-thigh length. I have three rain jackets -- (1) a Northface rain parka I can wear for outdoorsy activities and with casual outfits that comes to mid-thigh (2) a Stutterheim classic mackintosh that is a smidge longer and more professional and tidy looking -- good for work and (3) a Burberry-style belted trench (alas not actual Burberry -- I wish) that doesn't get that much wear but is good when I really want to protect an outfit head to toe and don't mind carrying an umbrella the whole time.

I also have a clear plastic rain bonnet that I've used a few times!

You don't need all that for a few months but just giving you a sense of how people who live in it prepare.

Need? Unless you live in the countryside and want to go for long muddy walks, or you’re going to Glastonbury or something, no of course not. I have the long and short ones and chose whichever looked better with my outfit when I lived there but I didn’t wear them often in London, it’s not that wet and it’s also not comfortable to keep them on indoors

You only need them in the country. If you're going on a city trip you won't be on muddy foot paths.

I’ve seen most feature the up to the knee wellie rainboot in social media posts but are these absolutely needed? Is there a reason ankle height is not more popular in England?