Hi Paul
Let's take PA and DA out of the equation here, for the time being.
My first question would be: Is your current website ranking for anything?
If it is and it is ranking well, I would question any site migration. "If it ain't broke, don't fix it" springs to mind.
With that aside - let's consider the user experience element:
"Will you have more trust to UK customers with a .co.uk vs a .com?"
As a UK business myself, I don't think that buyers have any preferential trust to one TLD or another - so long that it is clear on the website that you are UK based (UK numbers, locations, £ signs etc).
"Would a .com domain be more suitable for international clients?"
If you have aspirations to grow the company and target international regions, I would say that having a .com would be more suitable than a .co.uk - given that it is the global TLD. If this is a serious business consideration, that would make me lean more to the side of choosing .com.
"Will you rank better in Google.co.uk if you use a .co.uk domain?"
In short, no. In long, no - not really. Google uses many factors when deciding to rank a site well in a region or location - the TLD used is somewhere near the bottom of that pile. The other elements I talked about - UK address, numbers, spelling, server location, local citations etc - they will have a much, much greater impact to ranking. You'll see yourself that probably millions of .com sites are ranking well in Google UK.
I would look at all of those factors before I'd begin to think about PA/DA or the links a domain has. I'll be honest, it sounds a pretty poor suggestion from your last SEO company - particularly if they did nothing to justify their reasoning. Having a .co.uk domain is by no means a pre-requisite to rank well in Google UK. I'd ask yourself all of the above questions before reaching a decision on whether to switch or not.
Hope this helps.