What are some UK compatible alternatives to Shopify POS?

[Masikaluxe]

I shall be opening a brick and mortar store, so I have been looking into POS systems. Having trialled the Shopify POS for a few days, I find it quite glitchy and the interface appears to be lacking.

I am looking for an alternative POS system that that also syncs my inventory and sales with my online store if possible.

I would be looking to integrate with either sum up, square or iZettle card reader. I welcome advice and recommendations from anyone operating both online and offline.

Thanks in advance 

[IAClothing]

Have you had any luck?  I am in the same boat but in the US.