It really isn't, trust me, I've been through it a few times (as an employer).
There is a lot of detail hidden behind this:
"If you’re dismissed, your employer must show they’ve:
* a valid reason that they can justify
* acted reasonably in the circumstances
They must also:
* be consistent - eg not dismiss you for doing something that they let other employees do
* have investigated the situation fully before dismissing you - eg if a complaint was made about you"
There actually aren't that many valid reasons. Cultural fit certainly isn't one (and has the potential to get you in all sorts of equalities issues if there is any possibility that age, religion or anything else can be seen as a proxy for the way you didn't fit).
If you want to claim someone isn't good at their job you need to be able to back it up and show you gave them every opportunity to turn it around (including having given them all the relevant support and training). Generally speaking that would be a performance management exercise (several weeks of monitored performance) which is time consuming and rubbish for all involved.
And that's before you even get into the whole performance is hard to accurately measure in IT issues.
The investigated fully part is also important. If you diverge from your stated policy (or a reasonable policy if there isn't one stated - the ACAS one is usually the template) then you can be found against at a tribunal even if you reasons and evidence were sound.
Obviously people do ignore all this and get away with it (particularly in IT where people can often get new jobs relatively easily) but that doesn't mean you will or that what you did was legal.
Your best shot is if the contract has a defined probation period (3 to 6 months) during which you can pretty much get rid of people at will but after that, even before 2 years is up, it's going to take some work. It's not impossible but it's an exercise which will take a lot of your time and likely have a significant impact on team morale.