Sure ride it straight away, sooner you do the sooner you start learning.
You're going to be on a big learning curve for a year. First you have the wrong bike for starting in the UK but you may be ok in the dry of summer. Don't give up.
New plastics look good for a couple of rides. If you are new to dirt bikes you will be spending a fair bit of time in bushes or on your arse - actually if it stays dry you may get to spend more time on the bike than your arse!
After a year you learn how to fall and as long as you don't hit anything it becomes painless. Don't worry going down on dirt is usually a low sider and pretty painless. Oh I recommend knee pads of some sort though.
Don't worry if the plastics are just scratched. I would only bother with new ones when they really fell apart or if I was selling. First thing to do is to get the bike to fit you if you're not around average size and weight. That's the ergonomics - bar, pegs, seat heights, control positions. If short you can do things to lower a little. Then get the suspension set right. If not average get the correct rate springs fitted, set the sag (very important) and start reading about damping on dirt to help you setup the clickers.
Next check the jetting is correct. Dirt bikes often have the jetting messed about with. I'd recommend a CCK needle - look up Pobit's recommendations for the 300 using CCK for smooth power. You will find the 300 a very fast aggressive bike if you're new to dirt. Alternately go with a standard UK setup such as 38P, 178M, N1EF (on 3 or 2). Some US setups do not work here. I think they ride much more open terrain than us.
Tyres, tyres, tyres!
Almost anything will work ok on hard pack. When it's sloppy they get really important. I would advise Michelin comp 3 rears and comp 4 front. They are FIM legal for enduros and E marked for the road. After that the Metzeler FIM enduros are good too. Be warned the grip on tarmac is crap. I often forget when I've been riding road bikes for a while and end up in a speedway power slide on the first bend on a dirt bike!
Basically you need to make that bike as easy to ride as possible. Getting on a 300 2 stroke as a first enduro bike in the UK is a bit like passing your test on a 125 and buying a Desmosedicci as your first bike and going on a trackday!
You will be lucky to use more than 1/8 throttle in the woods. Makes overtaking on the fireroads easy though.