Does backlinks from international sites help UK ranking?

[SEOmm]

Hi,

I am curious that, if backlinks from those sites that are not indexed in 'page from the UK' search results can help a site's 'page from the UK' ranking?

Do you only trade or exchange links with sites indexed in 'pages from the UK'?

I have a PR4 site which has good ranking on google.com, And recently I created a sub-directory in order to rank on google.co.uk. So far, it seems that google.co.uk seems to think of this sub directory as a new site. And it looks like that it can't pass quality from PR4 home page to that sub-directory through cross site links.
This sub-directory takes 2 weeks to get indexed on google.co.uk. While as usual, if I give a cross site links to a in site page, that page should be indexed in no more than 3 days.

I am new to UK SEO.
I will really appreciate your advise.

[SEOmm]

Also I am interested in link exchanges / cross site 3 way linking with really good quality sites.
I have 5 quality shopping sites (1 PR5 & 4 PR4) which have good ranking on google.com. And 1000- 2000 unique visitors for each site.

And I would like to purchase PR3-5 site wide links about 'wedding / invitations / hair/ wigs/fashion ....' too. As well as blog reviews.