Just done some calculating myself.
I’d have to charge around £3.5 a order (roughly £1 a pick + £1.5 for 2Nd class postage to uk (Around 3-4 days). or £4.5 an order (£1 a pick + £2.5 (around 1-2 days)).
How much the postage costs is done on postage speed, weather you get compensation and the size & weight.
The £1.5 is for a smallish Jiffybag + 2nd Class postage up to 100 Grams with £20 compensation.
Postage would then go to £3.5 for something which requires a big jiffybag or box. Up to 1KG and £20 compensation.
A signature is then required for up to £50 compensation.
Compensation covers you for how much it cost YOU and not the buyer. I got tricked into this by paying for £30 (up to £50) compensation for an item that cost me £15 and the buyer £30.
The other route is that you maybe get a reseller in the UK which I’m also happy to offer.