Does anyone know if there is a free online Stock Screener for UK stocks, especially for the companies in the FTSE100? I'm looking for something similar to the Stock Screener created by Google Finance, which seems to work just for USA based stocks.
Yes, http://shares.telegraph.co.uk/stockscreener/ has what you're looking for.
Although this is an old question, it's worth pointing out that the Google Stock Screener now supports stocks traded on the London Stock Exchange. From the country dropdown on the left, select "United Kingdom" and use the screener as before.
Most free stock screeners for UK stocks, even those mentioned above, are very poor and not worth the effort really, and searching for stock screeners on a search engine will only bring up stock screeners for USA stocks.
The best free UK stock screener (registration is required although this is FREE) is without any doubt on www.digitallook.com who also provide many other features like five year fundamentals, charts, prospects, etc, which can easily be downloaded onto a spreadsheet.
I really wouldn't look elsewhere to be honest unless you are prepared to pay.
I use and recommend barchart.com. Again you have to register but it's free. Although it's a US system it has a full listing of UK stocks and ETFs under International > London. The big advantage of barchart.com is that you can do advanced technical screening with Stochastics and RS, new highs and lows, moving averages etc. You're not stuck with just fundamentals, which in my opinion belong to a previous era. Even if you don't share that opinion you'd still find barchart.com useful for UK stocks.
I know nice and free stock screener for UK (and 20+ exchanges) - https://unicornbay.com/screener?f=exchange_str|%3D|LSE;&s=MarketCapitalization|desc&p=1|20 from Unicorn Bay.
It supports both fundamental and technical analysis.
AdvFN has one--click the Charts & Research pulldown and choose UK Screener. Free but requires login.
I'm actually building a UK stock screener right now. It's more of an exercise in finding out how to work out technical things like MACD and EMA calculations, but if those are the things you're interested in, it's at http://www.pifflevalve.co.uk/screen-builder/
As I say, it's more of a personal project than anything commercial, but it's fun to play with.