Is there any difference between HL7 USA and UK standards ? If its there then what are those ?
I'm not aware of any country-specific HL7 features neither did Google query site:hl7.org country specific reveal something eye striking. But it does not mean there are no differences
I guess your best bet would be to narrow your question down to a particular set of messages, register as member at hl7.org and ask this question through their internal mailing list (Even better best bet might be to talk to some friendly competitor and ask for the lessons learned)
In our country-specific HL7 community the membership is also paid, but the benefits of becoming a member are close to 0 from the developer's perspective. It's prestigious to be a member and it looks good on the business card. But all you get is access to downloadable specifications (but most of them are free for about a year anyway)
In the software I was working with we were exchanging only very small set of hl7 messages, namely: ADT^A04, ADT^A05, ADT^A08, ADT^A18, ADT^A23, ADT^A34, ORM^O01 and ACK in 2 different countries, considering adding 3rd country, without any country-specific features needed, except the convention used for identifying patient with his/her security number. The number format was country-specific but it was handled by upper software layers. The topmost customization was the number input mask and user input validation business rules at GUI level
What we did need, was an installation-specific or 3rd-party-system-specific message customization. For this customization you'll need rather the list of vendors of hardware and software your system will talk to. Once you have it, check their hl7 conformance statements