My undergraduate maths degree was 4 years long and I got a MMath (master of mathematics), some of my friends did a 4 year chemistry undergrad degree, they got an MSci. BEng is the result of a 3 yr engineering bachelors degree, if you do the 4th year you have an MEng. If you're interested, having one of these 'M' undergraduate degrees isn't equivalent to having a 3 year BSc plus an MSc. The 4th year is the same length as the first 3 years, ie 38 weeks (i think), not a full 52 week year like a postgraduate MSc or MA. The main point of these degrees is for students who are interested in a career in academia, often having a MMath or MSci means they don't have to do a postgraduate masters before starting their PhD. This saves them some time and often money too as funding can be hard to come by for postgrad masters, and you don't get a loan with a very low interest rate like you do when you're an undergrad.
I think that this trend towards undergraduate masters is so far limited to the scientific subjects. sorry if this has bored you!