Hi Carl,
First, hosting location isn't a massive factor in terms of SEO - this wasn't always the case - however as sites are hosted in locations that they don't target (e.g. UK sites hosted in the USA) I think this is largely ignored. As such I'd suggest your hosting location shouldn't cause you major heartache.
In terms of the US vs UK question - it really depends on how much time / resource you have. If you have the time to devote to content creation, linkbuilding etc for both the .co.uk and the .com then great - if not focusing your efforts on one site and going down the subfolder route - /uk and /us is probably the best option for you. Do be careful not to replicate the sites in the UK and US folders though - you'll need to create separate, unique content for each.
Finally, re the IP redirect - this really isn't a tactic I favour. The issue with implementing a redirect based on IP is that as Google bot crawls from a US IP you can effectively render the UK version of the site invisible to bots. As such I favour the approach taken by CheapFlights. If you hit the .com version of the site from a UK IP you get sent to this 'choose your country' page - http://www.cheapflights.com/workers/profile-select.aspx?sref=CFUK&redirect=GeoIP&geoip=GB&cfref=CFUS&spt=Home&rp=/. This allows users to select the appropriate site for them and allows the bots to crawl both versions.
I hope this helps,
Hannah
