Best if you decide before hand. By the time you have decided on release time they will probably be sold out. With limited availability and MSI postponing their release till next week it makes an already difficult to obtain card even more harder. Have a back up option as well already decided. Chances are the one you want will sell out fast and they only have a miniscule amount of cards for certain brands.
Also as mentioned by
@Gibbo avoid finance and paypal for payment if you do manage to get the option to buy as they take longer to process and could possibly make your chances of securing one a moot point.
Also factor in the website will be hammered and it will slow down possibly go kaput under the traffic. All etailers websites will be under siege. The question is do you want to use that time on release to try and secure one on Ocuk or from another etailer. This time round OcUK does not have the largest allocation of 5090's. That choice only you can decide. But for me it wouldn't be a official launch date unless I chose Ocuk for the chaos.