Sorry if this has been asked before, i did search.
So I've sent some BTC to an address over 30mins ago and still not even 1 confirmation.
That can happen yes. Sometimes there is no block found for 30 minutes, sometimes your fee is to low and/or the priority of the transaction is not high enough.
Firstly wouldn't this be an issue for BTC to ever be mainstream if you can't pay for things immediately in shops etc?
No. Creditcards take 6-12 months for their first confirmation, I dont see anyone complaining about that or any customers camping 6 months because they paid for a coffee with a CC.
Also I presume the confirmation time will take longer the bigger the difficulty?
No, difficulty adjusts to keep the
average time to find a block at 10 minutes while computation power rises.
I think i've understood how it works correctly?
Im not sure how to answer that question, but I dont think you have understood how bitcoin works. If you have more questions feel free to ask.