I'd say £20 is well worth it, whereas I personally wouldn't pay £180. If you're completely unfamiliar with excel, I'd recommend taking the basic course at least as that will give you the building blocks you'll need to start with, and you can self-teach from there.
It depends on what kind of work you're likely to do - if you search online, you'll be able to find a list of what kind of "tasks" beginner, intermediate and advanced cover.
This will help with listing it on your CV as well - you can list it without having taken an official course, but what you put will give an indication of the kind of tasks you're capable of performing.
Eg basic/beginner would cover simple formulas, how to open and save a spreadsheet, simple databases and perhaps some basic charts. Intermediate might be more complicated formulae like Vlookups and pivot tables (an absolute life-saver if you're reporting on data!), and advanced would be things like vba, macros, array formulas etc.
As per pp, Google will always be your best friend with excel - you can almost always find someone who has already asked how to do the task you're trying to do, and an answer showing a good way to achieve your desired outcome.
You can also buy the "for dummies" books, as then you will always have a handy reference - though I'd advise trying to get newer books covering later versions of excel if possible as early versions don't have the same capabilities so you'll be missing a fair few how-tos if you get a much older book.
One tip which has really helped me, is finding out about the "fx" button. If you want to find out how to do something, Google can tell you the kind of formula you need, and then pushing the "fx" button in excel will walk you through writing the formula. (See picture).