OP, you’re streets ahead of someone who hasn’t even thought about pensions, or has but has then buried their head in the sand! Don’t be embarrassed.
Yes, you can pay in a lump sum.
I can phone my personal pension provider (I use Aviva, Hargreaves Lansdowne and Standard Life as I’ve got a few!) and just pay in giving my debit card number. It’s that easy! Others have mentioned the maximum (£40K pa or your annual earnings, whichever is lower, though there are some carry over allowances too).
As to the risk...
Yes, investment is risky. But when you choose a pension provider, you will also decide your risk profile. An IFA would ask you questions to help you work out how risk averse you are. That’s partly your personality but also your circumstances!
As an example, I’m a fairly cautious person. Quite “bird in the hand”. But, I have a work pension with a guaranteed value as well as my personal pension. So I’ve opted to go higher risk on the latter - because if the market crashes, it’s not the only money I am relying on. When I choose funds in the personal pension, I choose ones rated 4 or 5 out of 5 on Aviva’s own risk rating. Then, I let the Aviva fund managers decide where to invest it. They could buy high risk start ups in a new industry, or buy some government bonds with a very low but guaranteed return - depending on what risk level I have suggested.
Could you still lose on low risk? Yes.
But if your money is under the mattress, it will be degraded in value over the years by doing nothing. Not even a risk - that’s pretty much fact! So don’t be scared of the idea of risk. If you do nothing else with the “mattress money”, but Premium Bonds. There’s no interest, but you might win something!
Start by reading the Money Advisory Service website - pensions aren’t as complicated as you think before you start researching!