When it comes to the clothes I take away with me, less is certainly more. My bag generally has more gear in it than clothes because if I really need a pair of jeans, I can just buy them. Trying to find a UK plug adaptor abroad is always going to be harder. Here are a few essentials thats years of travelling have shown to be very useful things to carry with you.
Duct tape! - This stuff fixes everything and is an absolute must, it's saved my bacon so many times. Anything from holding plugs in sockets, to fixing a hole in your mosquito net. Great for jimmy-rigging all kinds of contraptions when you're camping.
Sarong - Probably the most versatile thing in my bag. It's a towel, a bed sheet, a scarf, a pillow cover, a pillow, a skirt, packaging for fragile stuff in your bag, a head and shoulder cover for visits to Mosques and Temples. Not bad for 50p!
Carabiners - Great for clipping things to the outside of your bag when you're hiking for quick access like a water bottle or a poncho if the weather is being temperamental. Good for clipping wet shoes to the outside of your bag too so they dry quicker and don't make everything smell like toe jam.
Rope - Weird one I know, but really useful to raise or lower you bag off something if you have to climb and you don't fancy the extra weight on your back. Also needed if you're camping in bear country to string up your food. I generally take two, one to keep long the other to cut when I need to string up a poncho. Also great for washing lines
Poncho - Another versatile item. Not only is it a rain cover for you but it will also cover your bag too. It can also be used as a waterproof sheet to sit on if the ground is damp and can be made into a shelter if needs be.
Multi-Plug Adaptor - Great for charging several things at once, even better when there's only 1 plug in your room. If you're roughing it and you pop into a cafe, even 30 mins of juice to all your essentials can make all the difference.
Playing Cards - Need I say more? An essential on all trips, especially camping. I think it's safe to say the whole world knows how to play shit head and kings cup so there's always going to be someone to play.
Fake Wallet - After being robbed in Mexico, a wallet with a few expired cards, a few USD and some local currency is always at the top of my bag. Hopefully it will keep any potential mugger happy.
If you're camping out at night though, check first to see what night time temperatures go down to before you leave because thermals might be an essential, even in unexpected places. When we cycled around Morocco it would be 30+ during the day but would get down to as low as 2 or 3 degrees at night, so taking a pair of thermals to the Sahara wasn't as silly as it sounded.
My mind boggles at the size of some people's bags, what do they have in them?? If we're headed somewhere really cold I can understand the need for lots of clothes and decent shoes or boots, but if you're headed somewhere hot what are you really going to wear? Shorts, t-shirts, dresses and swimming gear, and the reality is you wont be washing them all that often so you really dont need many of them. I normally take 4 t-shirts, 2 pairs of boardies, 1 pair of normal shorts and the flip flops on my feet, mainly because I don't want to be walking around in 35C with a 20kg bag on my back! Pack light and if you really need something, just buy it when you're there. I can almost guarantee you will not wear everything you pack on your next trip...