If its only you're fifth time in the surf, then a pop-out will do you fine. They're nowhere near as nice to surf as a 'real' board, but seeing as you'll probably progress on it then want to sell it, spending loads on a custom isnt the way to go.
So what do you want...
You're still learning so you're right to say you want something with a bit of volume, and you probably dont want a proper short board just yet. Trust me on this - trying to learn on a shortboard is a hugely frustrating experience. Particularly if you're not surfing everyday, and by the sounds of it you're not that close to the ocean?
And if you're going to be surfing wittering/bournemouth then a shortboard will be almost useless for you - the waves there are not nearly as powerful as croyde etc.
You need a fish or funboard shape (unless you go for a mini-mal, but if you're determined to be a shortboarder, then might as well start closer to a short board than a mal). They'll catch waves easier and give you the volume you need to.
So get something like 7'4 or 7'6, and 2.75" thick, and a funboard shape. There are several pop-outs that fit the bill. Then after a while, you can sell that, and get a custom, by which time you'll know if you need a shortboard or not.
But if you're surfing the UK south coast, then you'll probably want to stick to a fish/fun board.
Both fishes and funboards can be surfed at croyde, Puts, saunton etc and unless you're a surfing god, will get you a better share of the waves than you will get on a shortie.
Hope that helps!