If you access your email via a web interface, odds are the company logo on the website uses more kilobytes than all the emails you can type. The add banner will have 10 times more bytes than the emails you’ll send and receive.
If you open an email, and it has picture attachments, you may find that you have used a huge allotment of your 120MB. if you have email client set up on iPad, you may download those big attachments even if you never open them.
A typical web picture might be between 100KB and 500KB or half a megabyte. I browse occasionally on my iPhone and have trouble egtting to 50MB / month.
However, graphics and web sites on an iPad? You could hit your limit in a few dozen pages.
The real question is - what happens if you go over? Do you then get dinged another $30 or do they switch to the “super-highway-robbery” higher rate, or does it stop working? My guess is whatever gets the phone company most money.
The other poster had the best idea - suck it up and pay for internet. usually that’s $10 to $25 for an expensive hotel. Maybe buy internet every other day or less. If the problem is, no wifi- take a small wifi router with you, preprogrammed for DHCP as the WAN side of the router and a password-protected wireless. I used an Apple Airport. It works with 220V, all you need is a USA-to-British plug adaptor, which you need anyway for the iPad charger.
not sure if there are public Wifi spots in the UK while you’re out and about, but someone can tell us (I see they ahve).