Kickdown? I assume you're talking about down shifting?

I had a similar issue whenever I tried to take off, I felt like my car stopped pulling at around 90mph and wouldn't go past 110. I opened up the engine cover and turned out the intake was full of dirt and leaves. So I cleaned it out and that helped alot. Not sure where you live in England but it was pretty dusty where I was. Start with the cheap stuff first. I also changed out the spark plugs, I got autolite 2X platinum for $35 w/shipping from rockauto.com that seemed to help a bit and my mileage went up.

Whenever you buy a used car always:
Change oil + filter
spark plugs
brakes + rotors
Transmission fluid 5w-20 motor oil for manual tranny
Coolant
Brake Fluid

Changing out all of these will give you peace of mind and should anything be wrong with the car you'll notice it.

Careful passing on those single carriage ways, I had a dumbass hit me head on because she thought she could pass 8 cars at once.