You need to stay clear of the 900MHz band in the UK and lots of other countries, its mainly used for GSM (Vodafone & O2).
900MHz for ISM use, is generally only allowed in North & South America and Australia, some other countries may have adopted it also, if they don't use the GSM900 band.
For use in the UK you can use 433 and 868, although 868 was originally designated for use with RFID and alarm systems, it permits a 10% duty cycle so sending small packets of data from sensors should be OK.
Regards,
Leftfield95