(Things may have changed since this was my dayjob but here's my experience)
One of the Big problems of doing a new EPOS system is certification, Three agencies will show an interest
1) Weights and Measures if you are using an integrated scale
2) The Banks if you are taking Card payments (which of course you are)
3) HMRC for the VAT calculation
The weights and measures certification involved us making a trip to the National Physical Lab Teddington to demonstrate our kit with various test scripts to prove we could weigh,claulate and price transactions. The certification at the time was for a 'set' of kit The OS Our software and specific hardware.
.Once certified the package was sealed and you had to show how it could be verified as the tested kit (i.e. our suggestion was MD5 signature on the weighscale module of the software).
OH and don't forget to pay on your way out
(Brexit note it was possible once to certify in an more relaxed EU state but I don't know if this is still possible)
The Banks, Oh Joy; Data protection card protection etc well covered by EL Reg but the practicalities are again similar to the weights and measures.
You trot off to WorldPays offices (they are the only one left now aren't they) Demonstrate your kit, work through the scripts and get a pass or fail. Again you are limited to specific kit so another card reader company would require certification again, Plus when they dream up a new payment method such as contactless you have to go back and certify again.
OH and don't to forget to pay again (a lot) on exit each and every time you pass GO.
Finally HMRC; VAT is easy right price + 20% is selling price. Except in all those cases where it isn't second hand, kids clothing, energy costs, food take away\eat in, Jaffa cakes. Plus how do I round out the pennies. Anyone who's worked with HMRC knows how inscrutable they are .
ME: How do you want me to work this out
HMRC : Show me how you do it now
Hmm not quite right,,,,
Oh and how do you print the receipts ??
ME: (FFS kill me now )
And the final one of course, retail being that great cash generating business You could see the eyes glaze over every time you said to your customers; You'll need a new computer, Sorry you can't upgrade this one it's ten years, old knackered, you can't get the bits, and the memory isn't big enough, so you'll need to spend thousands to be able to take <£30 contactless..
That's why I no longer have that Day Job..