The tuition to a foreign school counts only if the foreign university is a qualified institution (see below for details). If University of London is not a qualified school for tax purposes, the CP2000 would indicate line 50 as incorrect. If the school didn't credit the payment until following year, but payment was made correctly, line 50 would show that as well. I recommend that you call the phone number on the letter (and be sure to contact them by the deadline) and ask. The IRS is usually very patient and informative when it comes to CP2000 explanation; they may even grant you more time to resolve the issue with them.
A qualified institution for education tax purposes is any college, university, or other post-secondary educational institution eligible to participate in a student aid program Administered by the U.S. Department of Education. Most foreign schools are not. Please refer to the U.S. Federal Student Aid https://fafsa.ed.gov/FAFSA/app/schoolSearch use this link to find a list of eligible schools and their FAFSA Federal EIN's. For the state pull down, there is the option foreign country.... Not all foreign schools will be listed, only a few will be missing who may also qualify. If they are not on the list, you need to then contact them. (choose foreign country in pull down) link to check if your university is qualified educational institution for tax education purposes.
