When I came over, I also had a box with my winter coat & some heavy sweater shipped separately (once I had an address). You can ground post these, so it's cheaper. Space bags ARE good. I've never had any of them ripped open - if they search your bag, I think you can pretty much see/feel that there's only clothes inside (they're clear, anyway). They ARE dangerous because they can almost be TOO efficient. I managed to cram everything in my suitcases with space bags, but then the cases were too heavy!! Weight was way more of a problem for me rather than size. In this case, my mom was too good of a packer and we got too much in the suitcase. Just try and keep things as light as possible. Shoes are really heavy, so sacrifice a few and just face up to having to buy some replacements in the UK (especially boots). Jackets/sweaters also apply, but I had these shipped because I didn't want to do without some of my favs.
Take a few things like photos or very small trinkets. They are light & don't take really any space, but you will be very glad you brought them!
I shipped my books via M-Bags, but this is because I've amassed a decent collection on my special subject & needed them for consultation/research. I'm doing a Master's, so it's a bit different. However, if you're doing undergrad, don't worry about books at all. You can buy £1 easy-reads at Oxfam, trust me (and then donate them back when you're reading to go home!).
But I always bring too much stuff, so not sure if you should listen to me! Hehe...