A tent - I like Decathlon ones, make sure you get one big enough that you can stand up in it. If there will be 4 of you camping, get a 6 man tent. Sewn in groundsheets aren't the be-all and end all a lot would have you believe, and help keep tents stuffy in warmer weather.
Self inflating mats to sleep on. The normal blue foam ones rolls are rubbish, air beds are rubbish, self inflating mats are amazing. A 2.5cm one is pretty good, a 5cm one is really comfy. Did 2 weeks on a 5cm thick mat a couple of years ago. Vango do good doubles, everyone (Aldi, Lidl, Argos, Wilko etc.) do singles for the kids.
Sleeping bags. Nice and cheap from decathlon again.
A table and chairs. Decathlon do decent tables, Ikea's folding chairs are bob on.
A cooker - get a one of those £10 single ring briefcase jobbies to brew up with if you're just trying it out. Spend £80-£100 on gas bottles and a proper two ring stove and grill if you like it.
Torches. Lots of torches.
If you're going for longer, try and lay your hands on a camp kitchen to stand your cooker on, makes producing food a far more pleasant experience.
You should be able to get completely kitted out for £200-£300 at the likes of Decathlon / Go Outdoors etc.