Right, I currently use Apache, PHP and mysql.

I've been looking around at some web developer jobs on the internet (UK), and most seem to want experience in ASP and Microsoft Servers.

So my question is, do many companies (mainly in the UK) use PHP and Apache for their web server and websites, or do they use ASP and IIS?

The reason I ask is so that I can maybe pay to do a course in ASP so that I have knowledge of it for the future if I ever want to apply for positions.

Moved to General

These are complimentary fields of knowledge and you would be well served to go ahead and learn both. There is even an implementation of the .Net environment you can run under Linux on your Apache server.

http://www.mono-project.com/Main_Page

I'm employed (in the UK) mostly writing PHP on Apache, but I have also been asked to work with ASP/IIS a number of times, so I can only echo what has been said already: Learn both.

Generally speaking, I've found that IIS/ASP is mostly used by in-house developers, where a company develops and hosts their own sites, whereas PHP/Apache is more commonly used where the company outsources their websites to a specialist ISP or development company.

Why is this? From what I've seen, it's simply that most companies who decide to develop their websites in-house already have an IT department with strong Microsoft skills, so they pick the solution they have the skills to support.
On the other hand, the specialist ISP has the skills to support whatever platform is required, but most of their customers don't actually care what platform they use, so they pick the one that is most cost effective or preferred by their development team, which usually means PHP.