Butter beans, cannellini beans, haricot beans, lima beans and Great Northern beans are all cultivars of the same species Phaseolus vulgaris which also includes other common beans such as red kidney beans and black turtle beans.

Horticulturist Oscar Will allegedly received the Great Northern beans from a Hidatsa Indian in the late 19th century. Will was instrumental in preserving some of the heritage cultivars.

https://www.camelliabrand.com/about-the-bean/about-great-northern-beans/
https://www.mypatriotsupply.com/Great_Northern_Shell_Bean_Heirloom_Survival_Seeds_p/great_northern.htm
http://ndstudies.gov/gr8/content/unit-iii-waves-development-1861-1920/lesson-3-building-communities/topic-7-telling-our-story/section-5-oscar-will

Unless you buy seed packets from the USA, it is unlikely that one would find Great Northern bean cultivar in the UK. The cultivar is commonly planted in the Great Plains states in the USA, but less commonly elsewhere.

Any white bean is likely a good candidate for replacing the Great Northern bean.

Are these what we English call butter beans? I.e. Bigger than cannellini/haricot beans ?

Navy beans or yellow eye beans.

Another alternative is baby limas.

Navy beans and Cannelini will do in a pinch, but I know a lot of people who will only use Great Northerns because they swear they make a difference in their version of baked beans. Personally, I use whatever is most convenient, my local Winco sells them in bulk bins.

Navy beans are a bit smaller, cannelini a bit larger, but I can't make a big difference in the taste. I tend to prefer navies, myself. But I can't imagine not being able to find Great Northern -- I can usually get them even if a store doesn't carry navies or cannelini

Yeah, I think any white bean will do.

https://www.wholefoodsmarket.com/recipes/guides/beans.php

She could try navy beans or cannellini. I imagine the bulk bins at Whole Foods would have them