I'm new to this stuff also, I found MMM and, from here other like minded uk sites about May of this year, believe me you're going to be doing a lot of reading, but like every thing as you go along you'll understand more and more. Here is as basic as I can put it. (I stand to be corrected if needs be)
You can't buy direct from vanguard with less than 100k, so what you do is go to a uk broker who takes many little/medium contributions from lots of people to make up the 100k and they buy the fund from vanguard on your behalf. The broker then charges a % fee for the privilege, bear in mind that vanguard also charges a % for running the fund.
I'm just at the start of my money building career and at 39 I'm starting late but "better late than never".
As advised above you should subscribe to monevator, he's a U.K investment blogger. Read through the previous posts from the beginning. Don't worry if you don't understand any of it as I didn't. The more you read the more you will learn. If after you have read loads it still doesn't interest you or you still don't understand any of it that doesn't matter as you will be taking a passive investing approach. This means you don't have to do anything other than put your money into the fund on an ongoing basis and vanguard will do the rest.
So from the monevator chart on U.K. Brokers I decided on cavendish as my broker. I opened an account with cavandish and chose a stocks and shares isa as my savings vehicle , the product I chose to invest in is the vanguard life strategy 80/20 acc( acc means your dividends are automatically reinvested into the fund).The vg Ls 80/20 with cavendish, as far as I’m aware the fees are 0.3% for cavendish(for buying on your behalf) and 0.24% for vg(for managing your fund) for a total of 0.54% of your portfolio per year.
I set up a direct debit for the same amount every month that goes into the fund and that's it. You don't need to do anything else. As your wealth grows you may need to move brokers to get better deals on fees as the lower the fees the better the return on your portfolio. Changing broker is like changing car or home insurance, you weigh up the options and decide of its worth moving for a better deal. But like me since you are at the start of your build you have time to figure these things out BUT in the meantime get your money into something.
Link to monevator U.K broker list.
http://monevator.com/compare-uk-cheapest-online-brokers/comment-page-26/#comments