There are many nurseries in the UK that would probably have a nice selection of standard roses to choose from.

Here is a link that might be useful: UK nurseries on HMF

I think what might be a better route would be to see what kinds of roses are offered as standards where you are, make a list of the red and white ones, and then ask about those particular varieties. It would be a shame for someone from the US to recommend a variety to you that is not available as a standard in the UK. Sure, you can inquire about custom propagation, but that will be more expensive (and require a longer wait-time) than simply selecting from those already offered.

:-)

~Christopher