Some or all of the following should help -
Find some way of restricting emigration while keeping other trends constant (some years ago I came across a projection that if something like this had been in place during the 18th/19th centuries Britain would have a population of 60 million by 1900 and over 100 million by 2000).
Find some way of avoiding WW1 or at least winning it quickly - 800,000 extra young men in the prime of their lives available to marry and have children in the 1920's should be worth an extra few million descendants by 2000.
Find some way of improving race relations enough so that restrictions on immigration from the (non-white bits of the) Commonwealth don't come in until an extra couple of million immigrants have arrived.
Slow down the development of women's rights. Fewer women in the workplace, more stay at home moms, more kids.
Note, not all of these (especially the last) will make Britain a happy place. But they will make it a more crowded one...