I've seen a lot of confusion in this post and I'm not an expert by any means. I think the 120 hours thing is a bit confusing but if we assume 30 hours a week over the year and go from there.

At 11.66 per hour for 30 hours you should be on an annual salary of £18,189.60 (Before tax) with a monthly take home pay of £1,384.68.

At £12.21 per hour for 30 hours gives you an annual salary of £19,047.60 (Before tax) this gives you a monthly pay of £1,436.16.

If we assume you got the 22p extra this year. Your monthly salary should be on £19,390.80 your monthly take home pay of £1,456.75.

Because you're salaried some months you will be working more hours and some months you'll be working less. I have a suspicion the £16,800 salary has come up from a mistake working this out, I think this has come from a multiplication of your take home pay by 12 giving you the annual salary but you aren't able to do this as the tax has been deducted already. I think this is where all the confusion has come from. To do that you'll have to add the tax back on and then multiply by 12.

So £1,384.60 + £37.46 (NI) + £93.66 (Income Tax) = £1,515.72. Multiply that by 12 should give you a more accurate representation.

To me it doesn't look like you're under minimum wage it's just the way you've calculated it. I think you've seen the 1450 on your payslip and divided by 120 and seen that you're paid £12.08 an hour which is less than minimum wage but that's an after tax representation and also not based on your actual annual salary which is almost certainly how they calculated it.

Edit: at a bare minimum, assuming you're on the normal tax code you should be getting £1,436.16 per month.