BTEC National Diplomas count for anywhere between 120 and 360 UCAS points, depending upon the grades. See here for details:
http://www.ucas.com/students/ucas_tariff/tarif ftables/
However, as you state, individual universities (or faculties within universities) are free to determine which qualifications they regard as relevant for entry onto their courses. The only way to be certain is to study the entry requirements for each course that interests you.
Search for relevant courses here:
http://www.ucas.com/students/coursesearch/cour sesearch2009/
When you find a course which looks interesting, click on 'Course requirements'. You'll probably then have to click on 'Course specific requirements'. That will tell you about any qualifications which are essential or, alternatively, about any qualifications which aren't regarded as valid for entry onto that course.
Chris