Sorry for such a dumb question, but we are older and don't understand this phone stuff as well as we should.
We will be traveling to the UK for three weeks this summer. The guy at our local Apple store (in the USA) said that the best deal was to go with Virgin Mobile for a 1 month contract. Thus,
You will likely need to buy the SIM in the UK but look on eBay or Amazon.
Your iPhone will need to be unlocked.
Check the website for iPad data SIM info.
Note: Virgin Mobile UK does NOT offer 4G/LTE data speeds.