I did MORSE in my first year. The maths modules are by far the hardest, and require a lot more work.
I switched to MathStat (very similar modules) in my second year as I preferred the maths over the economics, and for some techincal reasons regarding core MORSE modules.
Speaking from my experience as a first year, it was tough. I'm not going to sugarcoat that, you do have to work very hard compared to how it was/is at A-levels. But, you get used to the work load, and it gets slightly easier when you're used to it. Most of my friends are now on MORSE second year, and some have completely steered clear from the maths modules (they're not core in year 2/3) and opted for economics modules instead, they have found this past term very easy.
As for Southhampton MORSE, I'm aware they offer it but I can't really say anything about it.