I agree with what has been said so far. In my opinion, the average U.K. audience is comfortable with .com or .co.uk fairly equally although the .uk registry claimed 77% of UK residents preferred .co.uk over .com, but of course they'd say that. As for your example, I think a small business that looks up the .co.uk and .com and only one is registered, would go ahead and take whichever is available. A big business would take more consideration of various factors including those mentioned by others here.
The problem with selling .coms in the UK is that the sometimes preferred co.uk is usually much cheaper and far more likely to be available. Even with UKcityKeyword.com domains you are buying in a bigger, more competitive market than you are selling in because lots of UK cities are also city names in the US, Canada, and Austrailia etc. Birminghamflowers.com for example has been continuously registered since 2000 whilst the .co.uk since 2007. I guess you could turn that around and say you'd have multiple markets to sell in but I think its a tough business to sell localised .coms to small and medium sized businesses in any country with a well established ccTLD.
Your suspicion about keywordUK.com is spot on, I've seen maybe a couple of small businesses use it but investing in them would be like buying a lotto ticket where the odds are the same but the jackpot is only about $50. There is no aftermarket for them either, Namebio lists 24 .coms sold in the last year with UK at the end, with less than 10 being obviously for the UK.