Linking for visitors AND for SEO

Hi all,

I'm new(ish) to the Warrior Forum and I'm looking for a bit of advice......

I have two website with the same name one for US/CA visitors with .com ext and one for UK visitors with .co.uk ext

The sites are affiliate sites listing affiliate products within the niche but with different merchants in each country the sites are quite different.

I am trying to find a way of linking the sites so that any visitors who end on the wrong site for their country have an obvious link to the correct site.

I am looking at this from two points of view - visitors and SE/robots/SEO

1. Visitors - if you lived in UK and were looking for blue widgets and ended up at bluewidgets.com where all prices were in US Dollars. What would be the best way to catch your eye that we offered a UK version of the site.

a) A British flag in top right corner of screen - like a peel away ad
b) A Union Jack flag within the sites template
c) A flag next to the US Dollar prices
d) A text link next to the prices saying something like "UK Customers click HERE"
e) A combination of any of the above

2. SE/robots/SEO

If I have a link on every page of bluewidgets.com to bluewidgets.co.uk and vice versa, is this likely to harm my SEO efforts. Would a simple nofollow tag resolve any "penalties" and/or issues. Or can anyone suggest a better way to link the two sites

Thanks in advance
JoJo