I did computer science at the 3rd top UK university at the UK. I'm not a researcher there in mechanical engineering.
do Computer Science. any of the web developement stuff you can easily learn around it, but doing web development you won't get any job prospects at the end. Computer science has meant i can learn programming languages in a few hours, write any software i ever need and even program embedded systems to wake me up, make my breakfast and pour me a coffee.
Yes, there is a lot of maths. Deal with it. Going into that degree from a programming background, i found the programming boring and the maths side fun. I eventually did my dissertation in a theoretical area and didn't look back since.
You can always do a masters in web development.
You're losing too much in terms of skills and knowledge and time to do web development. You might aswell sit and learn php/mysql/htm5 and buy a few bits of software other than going to uni - you'll make more money and spend far less.
If you want something you can always fall back on, there will always be jobs, open many many doors and give you the skills needed to pick up web development, stick to computer science.
Oh, and any BCS accredited Bsc(Hons) in the UK has to do some web development these days AFAIK, so you'll be able to meet and find the right people quite easily.
in case you were wondering; my research is mechanical engineering - i do data mining for wind turbine failures. Nothing to do with my degree as we didn't even do datamining. It's the maths and theory part which got my the job!