the big question is what time of which day?
For public transport options go to
http://www.tfl.gov.uk - use the hotel postcode as a start point and Wembley Stadium "Point of Interest" as the destination. It will give various options which you can view in detail, if you get down to one specific journey click on the interactive map and then on "Wizard", it's worth it for the entertainment value!
For taxi quotes how about starting at the Yellow Pages website,
http://www.yell.com which will also allow using the hotel postcode to narrow down the search.
If it is for a busy event with likely traffic problems consider combining the two, get a taxi to Uxbridge Underground (or South Ruislip mainline station) and then go direct to Wembley from there.