1 with a 'dedicated' portable wireless router (a "MiFi" or mobile Wi-Fi, I suppose was their thinking) from one of the mobile networks. The device is a wireless router, and 3G 'mobile' unit, with battery which lasts for say 4-6 hours (I don't have one, but some other MNetters do). They're far from cheap at about 60-100 quid, but then again, neither is the alternative... I know Three, Vodafone and EE have such devices.
You can sometimes find people selling them on Ebay and I'd suggest looking for ones which are either unlocked, or on Three. I have a home unit from Three which was about 40 quid, but mine is just the wireless router part, and has a USB slot for a dongle to go in. Why do I suggest Three ? Only because the cost of data is usually lower.
Three sells (as brand new items) USB dongles with 3 GB or 12 GB pre-loaded SIMs. The 3 GB SIM will work as a PAYG SIM, in the sense that you've paid for 3 GB and it will allow access either (a) until you have used up 3 GB, or (b) until 3 months after the first day you use the SIM. The 12 GB SIM will run until 12 months after. However, of significance is that the 3 GB SIM may be found on Ebay for about 13-15 quid which may be enough for all your summer trips, and then you buy another next year... (or in 6-8 months, perhaps). I've seen the cost of these go from 8-9 pounds up to the current price, but I would not suggest buying 'several' in case they have some form of 'expiry date' and you're left with 45 quids worth of unusable SIMs in 3 years time...
Do watch out on Ebay though, because there are 3 GB Three SIMs and other listings for micro SIMs (to fit directly into a tablet). Probably worth asking the seller of a s/h MiFi what size SIM it needs... a brand new MiFi may be worth considering, but not on a contract, as a PAYG...
I am assuming the MiFi takes the normal size (mini) SIM. NB A full size SIM would be credit card size. I used to have such a SIM for an old Motorola on Orange in the 90s.