There are a few ways to go about it. The local Meetup group friendly tabletop games of Essex does one shots. Boardroom and a few other game stores do as well. Maybe ask around at the St Albans game stores?

I realize you asked about a campaign, and the answer is it's probably going to be difficult to find a DM. If you want to play now, best to go on other DND subreddits or discord channels and post in lfg, Roll 20 also has something similar. Foundry VTT discord also has a lfg. This way you can play online.

The local Meetup also has a specific DND discord.Meet-up D&D

If you want to play locally, make some friends at one shots, find people you like to play with, and go from there. Best of luck finding a group!