To answer your questions:
1. If you have the skills and experience you will be asked to attend an interview even if you are currently based outside of the UK. Some large multi-nationals will even pay your fare if you are really what they are looking for, though this will depend on the job you are applying for - it would have to be quite a high grade and the company would be intending to impress you in order to secure you.
2. The larger companies do seem to, this is an area that has expanded recently and is considered good HR practice in multi-nationals. Smaller companies don't seem to have the resources to hold the tests or the staff specifically trained in which tests to use and how to evaluate the results.
3. I have never worked at a company solely involved in IT, only large companies with IT departments but believe the answer is probably similar to 2, large multi-nationals and international companies will use these tests for practically all positions, smaller ones won't, although obviously hard skills and experience are more important than these tests. The company will tell you that you will be taking psychometric tests when they invite you for interview, they won't spring them on you unexpectedly.
4. There are loads of free psychometric / aptitude / personality tests on the internet, just type in the above words to any search engine and you'll see! The best advice to remember is that these tests are designed to test whether you are best suited to the job psychologically / emotionally, if you're not you probably wouldn't enjoy the particular demands involved anyway, so just tell the truth in your answers. There is no wrong / right, just different types of people. Only if you have extreme differences from what is 'best suited' would you lose out on a job because of psychometric testing, and then you wouldn't have enjoyed the role anyway.
One good site for general information is Prospects, this is the career guidance website for UK University students / recent graduates;
http://www.prospects.ac.uk/cms/ShowPage ... F;$A7$08m$
This page also includes a link to the British Psychological Society, loads of info about psychometric testing.