No agency is able to conduct a survey across all 60 million people living in Britain.
No one is suggesting that, but it's a big jump to extrapolate from 1000 to 60 million. They also provide no details of the demographics sampled or how they chose participants. If I survey 1000 users of this forum, I would find 100% have heard of bitcoin and 99% want more widespread usage. If I survey 1000 nonagenarians, I might find 1% have heard of bitcoin and none of them care about more widespread usage.
The important point of a sample is that it what's call a representative sample. Their ages, sex, race, education level, income, etc., should be in roughly the same proportion as the general population. Without this, an extrapolation is meaningless.