I don't have a good answer to this I'm afraid, but I can give you a few thoughts.
- A .co.uk domain shows explicitly that you are UK-based, which is where you are trying to sell stuff (in preference to offering free content). OTOH many UK-based businesses use .com addresses, and I'm sure you would say something like "Outlook and Word training in Buckinghamshire, UK" on the page too.
- While a UK domain may be a little strange for overseas visitors, if your content's good they'll read it. Recently there has been quite a profusion of 'odd' regional domains, such as bit.ly,
anything.tv and (going back a bit further) del.icio.us.
- Google uses regional domains to order results. This means a .co.uk address is likely to show up higher in a UK search and lower in (say) a US search. It's possible to set the location for .coms in Google's Webmaster Tools, so a regional .com is ranked the same as a .co.uk.
[Edit: Oops. Too slow posting. Greg has already said this.]
I guess what I'm saying, in a rather roundabout way

, is that if your content's good, you've stressed where you're based in the copy and you've done a bit of SEO then it really shouldn't matter which you choose. On balance, I think I'd go for the .com and redirect the .co.uk.
Anyone know more about this?