Low - national minimum wage, zero contract hours, no company benefits, no paid holiday, no extras
Medium - I think the UK average used to be £26k and up to £40k, so I would add employers that provide other benefits like pension contributions, paid holiday, healthcare, reduced services, share save etc
High - £41k plus and employers that provide company benefits
Your partner can put their job title into a search engine and find out an estimated salary
However, some company benefits are worth alot, so it is worth taking these into account eg pension contributions, healthcare
You also have to take into account things like commute to work, does company allow working from home, flexible working etc
You may choose to do a job that you love, rather than a job that pays a lot
A job is not always about the salary
Same salary range for self employed