..."£18,000 per annum OTE Weekly Pay"... means £18,000 On Target Earnings - so you will earn £18k IF you meet their targets

Generally this is a BIG if. Most companies set unrealistically high targets, so few, if any, achieve them.

So NO it does NOT mean £18k a year regardless. If it means that it would say £18k plus bonus or similar

It means you can earn 18K a year IF you meet their targets.

I wonder how many do.

yes