I'd be willing to believe that national laws may have some impact and that I won't know enough about UK law to address that part. But I'm also willing to bet that there's some general strategies that apply across the board.
Nature of the Merger
A lot of the dynamics will depend on the nature of the merger. In particular - how eager is your company to be part of it? For many small companies looking to grow, being merged or acquired is part of the game plan, in which case, your company may well do a downsize or other strategic reorganization before the acquisition goes through.
In other cases, if your company is unwillingly acquired, the layoffs will come after the merger or acquisition, as an outside entity can't force the change.
Size of Organizations
Needless to say, big organizations move more slowly than small ones. The sheer number of employees is a big factor here, as organizing a layoff takes more time if there are more people. In a really big company, it can be years before the big layoffs come, as they will take a LONG time figuring out what is actually redundant.
In an enormous company merger, I would likely not expect much change to happen before the paperwork is signed, and afterwards I'd expect very little change in the first year, unless there is a dramatically obvious and very expensive redundancy.
What are they buying/merging you for?
Mergers have different flavors - and a competitive one is particularly tricky. Are they trying to get your company out of the marketplace? Or are they tryng to add a feature or suite of product/service that gave your current company a competitive advantage and it will offer the new merged company a new revenue stream?
These kind of things will make a big difference in how rapid and dramatic layoffs will be. The trick is, I'm never willing to believe the line "we purchased you for your brain trust - it's the people that make you and asset and we respect that." - it's too soft a phrase and too easy to forget about later when you can't meet the bottom line. The big question is how quickly can the acquired group become an asset. Which is probably a question for your management.
Individual Employee Agreements
Usually as part of a merger, there is a recomputation of employee/employer agreements. Taht usually doesn't happen quickly, but at least in the US, it's part of the merger negotiation. So your 3 month notice requirement will hold for at least a little while, but you can expect that it might change, particularly if you have a sweeter deal than the standard industry expectation in your area. I've seen negotiations include a hands-off period of 6 momths to 1 year - where the new company was not allowed to change any employee benefits - but at least in the US, that's a matter of the contract between the organizations.
Being in a new hire position is particularly tricky because if your employment agreement includes a trial period where you can be let go with minimal paperwork if you are not a "good fit", you can be in a tenuous position. Since these sort of arrangements are deliberately designed to make it easy to let go a new person when they haven't lived up to the expectation set in the interview cycle, it is deliberately rather vague and aimed at saving the company the hassle and cost of the formality that you would otherwise be entitled to. Which makes is also fairly easy for them to say "oops, our needs changed, we don't need you anymore".
Yeah, but will I be laid off?
Thanks for not asking that one, because it would be the classic "it depends" - any given individual in any given company is going to be unique. I've seen plenty of layoff situations where the long-term 'we can't imagine living without him, he knows everything' guy got laid off and the newbie with less than 6 months time and no product knowledge was retained. Why? Because in that sitation, the long-term history products they wouldn't be selling didn't really matter, but the fact that the new guy had a depth of experience in a highly used technology made him the key ingredient for the new world order.
You never really know - it's just helpful to havea look at why the merger is happening, what the new company's strategy might be, and how you'll play into it. The company's goals for PR, legal requirements and company culture will play into it as well.