Well when it comes to digital files and stuff finding an independent one for Bitcoin could be possible but not right now I don't think, I've tried, like I said a lot of them are third party, so you'd have to hire a programmer to develop you one. There is absolutely no way you're going to get a seperate payment plugin or anything for dealing with GBP/USD/EUR because their systems aren't set up like that, it would be possible to do wire transfers but you'd of course still need a bank account for that.
Yes, there are options that do not require registration. Easy digital downloads(a wordpress plugin) has "guest checkout". I believe you can set up woocommerce in the same manner and sell as a "virtual" product. Or an alternative option would be to just use satoshi box. Also, magento can be configured for virtual items, however it is memory intensive and requires a very good server.
