I don't have twins, but as you're considering what you have to lug to the UK and since some have mentioned an umbrella stroller...I'd mention that while traveling my then 2 year old daughter, I purchased an umbrella stroller in Europe and it was the BEST! At the time, about twice the price of a cheap US umbrella stroller and totally worth it. We tried to travel to London and Paris without a stroller but just couldn't do it.
I also agree with PPs that all babies are different. We traveled when our kids were infants and had some lovely vacations. Congrats and best wishes!!