Re: so much wrong here...
Zombie topic? Going to reply anyway.
2 x 2.5mm cables in a ring main significantly exceeds the tolerance for overloads of a 2.5mm or 4mm radial circuit.
2x 2.5mm does "greatly exceed" 1x 2.5mm, but the former is protected at 32A while the latter is protected at 16A or 20A.
2.5mm cable is only "good for up to 27A" if installed effectively in free air, or with one side open to the air and the other clipped to a heat-conducting surface. Practical cables run in, on or through insulating material for at least some parts of their route and so must usually be rated at their lowest capacity. 2.5mm cable is good for 21A under (nearly) all circumstances, and this is why radials in 2.5mm are protected at no more than 20A.
Note that even a 20A breaker will not trip for small amounts of overload and could actually take several minutes to trip for moderate overloads.
You have two fault modes - moderate overloads (i.e. just too many things plugged in) and short circuits - but the critical information is the amount of power "let through" before the fault is cleared. Too much "let through" can damage a cable by overheating, and such damage may not be immediately obvious, and almost definitely not visible. Typically the conductors will begin to migrate through the PVC insulation.
I suggest you get hold of a copy of the regulations and look the appropriate figures up in the tables but very, very simply, 2.5mm cable is usually fine behind a 20A MCB for both moderate overloads and short circuits, but it is emphatically not fine behind a 32A MCB unless it is in an intact ring when you are essentially correct, it's similar to having two lots of cable in parallel. That's only really true if the load is evenly distributed - i.e. the fault is at the centre point of the ring - but most rings aren't large enough to cause too many problems.
There are two key points to make. Firstly, a broken conductor in a ring circuit will not be obvious at all, unless someone does a proper inspection on the circuit (every 10 years is recommended for domestic circuits I think). Other than a break in the earth, a radial circuit will stop working beyond the break and so your typical householder will probably investigate. A cheap plug-in tester will detect a broken earth in a radial circuit, but it will not detect any problems in a ring circuit.
Secondly, the earth conductor in most "Twin and Earth" cables is smaller than the live conductors. In a "2.5mm2" cable, the earth is actually 1.5mm2 and protection has to take this into account, because the most critical type of fault is a short circuit to earth through this conductor.
Lots of caveats, of course, and you might like to consider that pretty much all circuits are now required to be protected by an RCD, which has two key safety effects. Firstly, the RCD will still trip if there's a fault to earth, typically through a person, even if the cable's earth is broken, and secondly the RCD is usually much more sensitive and faster-acting than an MCB and even in the case of a fault to "normal" earth, will probably cut off the power well before an MCB would.
I'll say it again, I believe the way electricity is installed domestically in the UK is probably the safest in the world, and I certainly believe that our plugs are the best (unless you happen to stand on one in bare feet) but it is not perfect, it is not infallible, and there are many misconceptions that need correcting :-)