You should check out Three HK's unlimited roaming eSIM.

$138 HKD is about $17.50 USD at the moment. That will get you 10 days of unlimited data roaming in any of the supported countries (UK is one of them). 500MB per day is at full LTE speeds and then throttled until the next day if you go over. The 10 days don't have to be used consecutively. So, you can buy this and set it up at home to test before buying more than one eSIM account. You will blow one of your 10 days, however.

Here is a first-hand account of someone using this eSIM plan on a Google Pixel 3.