Walk up to a range table and you will see a row of replicas that all look broadly similar and behave completely differently. There are three ways an airsoft gun can work, and knowing which is which tells you most of what you need to know about how it will feel.
Spring
The simplest mechanism there is. You cock the gun by hand, which compresses a spring; pulling the trigger releases it, and a piston pushes a column of air that fires the BB. One shot, then you cock it again.
Feels like: deliberate. Because you cannot rush it, spring guns encourage you to actually aim, which is why they are often the best thing to hand a complete beginner.
Upside: almost nothing to go wrong, no battery, no gas, unaffected by cold. Downside: slow, and the cocking action is heavy on some models.
Electric (AEG)
The workhorse of the sport. An AEG — automatic electric gun — uses a battery and motor to drive a gearbox that cycles the same piston-and-spring arrangement automatically. Hold the trigger and it keeps firing.
Feels like: the one most people picture. Consistent, quick, and satisfying in a way that makes people ask for a second magazine.
Upside: reliable, consistent shot to shot, works in any weather. Downside: batteries need charging, and the internals are more complex.
Most rifles and SMGs you will shoot on a range are AEGs.
Gas
Gas guns use compressed gas — usually green gas or CO2 — released from a reservoir in the magazine. The best of them are gas blowback: some of that gas is used to cycle the slide or bolt backwards, exactly like a real firearm.
Feels like: the closest thing to the real experience. The slide kicks, the gun moves in your hand, and the empty magazine locks the slide back. Almost everyone's favourite once they have tried it.
Upside: unmatched realism and feel. Downside: gas pressure drops in cold weather, and they use consumables.
Most pistols and revolvers on a range table are gas.
Which should you ask for?
If you have never done it before, start with an electric rifle. It is forgiving, consistent, and lets you concentrate on aiming rather than on the mechanism. Then try a gas pistol, because the blowback is the thing people talk about on the way home.
A note on the UK rules
Airsoft in the UK is regulated. The law sets maximum energy limits for guns to count as airsoft rather than as firearms, and there are restrictions on buying realistic imitation firearms outside of organised airsoft activity. On a supervised range none of that is your problem — the replicas are the operator's, and they never leave the site.