being a person with both a BTEC and A-levels, I can't say I noticed any difference in the difficulty of them.
Although my BTEC is an Art & Design foundation, not sure if that counts as the kind of BTEC people are referring to in this thread? about 80% of the people on the course were straight from A-levels, with the aim of going to university after the foundation, which is only 1 year long, and can be done as a year 0 at university as oppose to at a college if somebody wanted to do that. I didn't fancy paying for an extra year though when I could do it at a local college for free!