How to gain admissions to graduate school with postgrad diploma, but no degree?
I can't vouch for the UK, but here on the other side of the puddle, once you've been out in the Real World (TM) for some number of years, your classes start mattering less and what you've done with them starts mattering more. A school's concern, if you're coming back for an advanced degree, is whether you have enough knowledge to cover the prerequisites and enough aptitude to complete the degree in a reasonable amount of time with a reasonable grade. (In other words, whether you'll be a student whom they're good for and who makes them look good.)

For PhD there may be some bias toward folks who are already researchers, but "research" comes in many flavors, not all academic.

(I could have gotten into Columbia in NY for a MS simply on the basis of "you can't be an idiot or IBM wouldn't have hired you" -- plus having aced a few of their extension classes. Can't imagine things are very different there.)