What's the most reliable estimator for a postcode?
BT's speed checker will give you a reasonably accurate range of speeds for a particular address:
https://www.productsandservices.bt.com/products/speed-checker/Click on "I don't know my landline number" to get it to work for an address only.
We generally get about 5 Mbps (we're a long way from the cabinet where the line comes from to our house), and the speed checker estimated we'd get between 3-7 Mbps.
And what does 76MB from Vodaphone mean? DSL?
Yes, the vast majority of people get internet via DSL through their phone lines. The lines are all maintained by a company called Open Reach which used to be BT. While you have a wide variety of providers, they're all using the same lines, so one of them isn't any faster than any of the others. The only differences are price and customer service. If you use a website like Uswitch or MoneySavingExpert you can see which providers are available in your area and what packages they're offering.
Speed depends on how much of the line to your house is fibre or copper. Some places you get fibre to the cabinet that serves your area and then copper to your house (that's what we have, and it's never going to be upgraded to fibre to the houses because there are too few houses where we live to make it cost effective). In other places you get fibre all the way to the house, and speeds there will be considerably higher.
Some people get the internet through cable using Virgin as their provider, but Virgin doesn't cover as many areas.