Rear Brake Fluctuating

[fruitsalad]

But not excessively this is an advisory on my car, what does it mean?

[Zacs-Master]

It means that if you take it to the same place next year, they'll probably fail it on that, costing you a few quid.

It actually can be a sign of a warped drum or disc.

Get a second opinion.

[fruitsalad]

It cost me enough to get through this year's Zac

[Hymie]

I had this as an MOT failure recently (but on a front wheel).

The fault was identified as a sticking brake caliper (on one side) resulting in the fluctuation with only one brake pad pressing on the disk during braking.
The garage fix was to un-seize the caliper with brute force.

In my opinion, the seized caliper was due to my driving style, anticipating the traffic and using the brakes spearingly – with little use, allowing the parts to seize.