Hi Nick, and firstly welcome to CruiseMates.
Wow Queen Victoria for your first cruise, very classy

) and so to your questions.
1. Britannia - the dress code is only applicable in the evenings, for other meals it is as casual as the self service Lido, normally people would wear long trousers/pants & short sleeve shirts or tee-shirts
2. yes, and yes ;o) there are 2 sections in the room service, one with snacks, sandwiches, soup etc, and another where you can order full meals which have the price shown next to them. Tipping is of course at your discression, but certainly not expected for room service.
3. As it's only a 4 day cruise, the age will be pretty much around yours, from mid-20's through late 40's with a few more seasoned passengers grabbing the chance of a short break.
4. For ships sailing in & out of European ports, this doesn't seem to be enforced - it is there as a necessity so that it can be enforced if felt that party groups could get outr of control. The worst thing that can happen is that they take any excess away from you & then return it when you finally disembark. (if this happens, remember to reclaim it as instructed, or you will lose it)
Have a fantastic time on board, and other questions feel free to ask.
Alan & Katrina