Welcome to the Forum!
CGT Exemption only becomes interesting when you are taking a considerable profit / building a very sizeable stack. I'm not a UK resident but if I recall correctly, the threshold of CGT exemption in general is about 10K pounds per year. This means that you'd have to have over 10K pounds ''profit'' before you get slapped with CGT and this resets annually. A common way around this is if you indeed have such amount of precious metals is to sell off no more than that 10K threshold in profit within the same (fiscal) year.
If you are not going to be close to that number, I would recommend getting whatever you want as long as it suits your strategy and budget. If you are going to be close to that number, you should definitely consider CGT exempt coins, so Brits and Sov's are your go-to items in that regard, but I'm relatively sure that any UK (pound) denominated coin is CGT exempt.
When it comes to selling, it depends on what you are selling on how easy it is to sell and where best to sell it. In my experience, larger items (for example bullion 1oz gold coins/bars) are easier to sell (back) to dealers than they are on second hand market unless you have a good trading reputation. You may not get the best price but you will be able to liquidate. Smaller items trade here on the forum all the time - have a look at the trading section and you'll see loads of 1oz silver coins and smaller gold pieces like 1/10oz, 1/4oz / sov's trade frequently.
I think you should answer the question ''where can I sell this when I want to sell it'' before making the majority of your purchases. Hopefully you'll never be forced to sell by any external factor other than your own decision, but if something does happen and you need to liquidate, it will be beneficial to have an idea and understanding of how you are going to liquidate your metals. In that regard, it's again advisable to obtain items like sovereigns or brits, especially in the UK, because they are well recognized and desired. It's easier to liquidate 2 bullions sov's than a 1/2oz gold coin (generally speaking).
Hope that helps and enjoy the journey, stacking metals really is a rewarding (and sometimes frustrating) hobby!