While I don't do Acturial Science I know 1 or 2 people who do. The main issue with direct entry in 2nd year for actuarial science is that you can't do a 1st year module called "introductory macroeconomics" which gives one of the exemptions if you score well enough in it. You can still get the other 7 if you do "finance and financial reporting" in 2nd year instead of "numerical analysis".
If you stay in halls then most other people will treat you like a first year so you don't really miss out on any of the social aspects. I believe actuarial science has quite a few people who do direct entry each year but I can't give you exact numbers. The only issue I've heard of people having academically is that the statistics in 2nd year can get hard quite quickly if you haven't done enough previously.
EDIT: At Heriot Watt, rest of UK students (which you are if you're from northern Ireland) only pay for 3 years if they complete a 4 year degree here so that you pay the same price as you would at a university in another part of the UK.