Fitting aftermarket wheels is legal in the UK and very common. But there are two things worth getting right, and both are simpler than people fear.
Insurance: declare them
Aftermarket wheels count as a modification, and UK insurers expect to be told. It takes a phone call.
The cost is usually small or nothing at all if the wheels are a similar size to standard. Going significantly larger can attract a premium, since bigger wheels cost more to replace and are associated with higher claim rates.
The reason to bother is not the premium. It is that an undeclared modification gives an insurer grounds to reduce or refuse a claim, and that applies to the whole claim, not just the wheels. If someone reverses into you, you do not want a conversation about your rims.
Tell them the size, the offset and whether you also changed the tyre size.
MOT: what the tester checks
There is no rule against aftermarket alloys in the MOT. What a tester will fail is:
A wheel that is cracked, badly buckled or has a damaged bead seat. A wheel that fouls the bodywork, suspension or brake lines on full lock. Tyres that sit outside the arch, since UK law requires the tread to be covered. Missing or incorrect wheel bolts, or bolts with the wrong seat type. Tyres below 1.6 mm tread or of mismatched sizes across an axle.
Spigot rings are not an MOT item, but a wheel that is not properly centred can cause vibration a tester will notice.
Bolt seats matter
Wheel bolts come with conical, radius or flat seats, and they are not interchangeable. Using the wrong type means the bolt is not clamping properly, however tight it feels. Aftermarket wheels often need different bolts to your originals.
Torque them correctly and re-check after around 100 miles.
The short version
Ring your insurer, use the right bolts, keep the tyres inside the arches, and you will have no trouble. Every wheel we sell states the size, offset and bore so you have the figures to hand when you call.