If I understand you correctly you are wanting advise on
A. what you need to improvise things and
B. what can be improvised.

The answer to A depends more on where you are, the UK being rather densely populated I would consider you being in a more urban environment, that being the case I would be sure to have a good multi tool and something to cut metal like a small hack saw and scissors that can handle light sheet metal. I have some fiskers game shears which are small light and will cut most things with in reason.

The answer to B would be a never ending list, for example you can boil water in a milk carton and apply and enormous amount of pulling or lifting force with a loop of rope and a stick. you are only limited by what you can think of when the need presents itself. It's just a matter of knowing what has to be done and seeing how you can use what you have to do so. The better you mind works at that sort of thing the less stuff you need in your pockets and bags because if you think about it the people who thought of those had neither.

The bottom line is the best tool you carry is the one between your ears. The people who use it survive, evolve, create... those who don't, don't

Jim