I'm a bit late to the party as you posted this in March haha. But you're gonna have a blast! Half of my Top Deck bus was Aussies, so we all connected well. For Europe make sure you have heaps of euros for toilets! In London eeeeverybody jaywalks, don't do it in Europe cause they crazy mofo drivers! Take earplugs in case someone snors. I just got Therafirm knee high flight stockings..the socks are too warm for me. Just measure your ankle and calf before you go buy them. $15 at chemist warehouse atm (unsure if they're all over Aust, they are in Qld). The Topdeck bus WiFi can be a bit dodgy, I had a Vodafone Red sim and paid $5 extra per day on top of my normal plan to use it overseas (including the data on my plan). Everyone was having issues with the bus and hostel WiFi, meanwhile I was sweet with my fast mobile Internet and 12gig data. Vodafone is all over Europe and London so it works wonderfully!
Take scented garbage bags for dirty clothing. Try to pack as little as you can cause you do have to lug your bag to and from the bus often. A 20,000mah power bank was a saviour for my phone. Try to do as much of the additional activities with the group as you can, otherwise you just feel like the odd noob out lol. Wear good walking shoes cause you'll be doing a lot of walking, I wore my sneakers every day. Black Milk high waisted velvet leggings under jeans saw me through snowy Switzerland. 'twisty turbies' were perfect to wrap my wet hair in after a shower and were really small in my luggage compared to taking a large extra towel just for my hair. If you have coloured hair like me, take a pillowcase so you don't stain the pillows you sleep on (and the headrest on that bus is now stained blue from my hair..whoops! ). Get a cross body bad for your valuables and keep ur hand on your bag at all times. If you're a fan of Black Books you can go to Bernard's bookshop in King's Cross. Go to the Camden Lock markets they're amazing. I met Noel Fielding while strolling around Camden! Go to Buckingham Palace at dawn, it will be empty and you can get stunning photos. See a show at a London theatre, I saw Wicked and it was epic! A hop on hop off bus tour of London was super handy to do for a few hours.
OK I'll shut up now hah. If you want, join a travel group on Facebook called Girls Love Travel. We are about 350,000 girls from all over the world. It's a wealth of knowledge.
Last but not least, have fun and be safe!