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Like I opined earlier, any of them will be "safe enough" if you use a
*fused* RCD connection unit. The 30A ring seems like the least loaded
circuit of the three, though I'm not quite sure what your "boiler" is
(a rice cooker? a Burco?); even if it's a 3kW jobbie, your 30A ring
will deliver 7.2kW, with the other appliances pulling under 2kW. So
that's the circuit I'd tap into. From the further details you give,
I'd firmly *not* use the 20A radial, as the w/mach and t/drier (which
could well both be on at the same time) will be eating maybe 4kW at
peak (though to be fair, w-machines spend relatively little of their
cycles heating water) leaving only 1 kW or so for your monster 4-way
Athlon-with-liquid-cooling computer setup ;-) and thus naff all for the
outside loads.

The boiler I mentioned is a Baxi central heating + water boiler (Baxi
WM 38 3RS ?). I have tried to look up the wattage for this model but
can't seem to find it - I am assuming it would be between 130-190
watts? Also I've calculated my computer uses around 400w (including a
print and scanner) but who knows what other gadgets I will buy in the
future....

It's possible that the sink has been moved at some point, or just that
someone got iffy about having a socket right next to the sink, as the
blanking plate+choc-box suggests that there used to be a socket in that
position.

I've found out from a neighbour that the kitchen was entended a few
years ago thus would explain why the electrics are so close to the
sink. Still, I thought junction boxes should be used instead of
connector blocks?

The two wires at the CU are indeed puzzling, and it'd be worth
tracing the Other one (since you know that just one of them feeds the
kitchen-and-back-bedroom run, right?) - initially by simply disconnecting
one of those two wires and seeing what, if anything, goes dead. Remember
it's still possible this 20A circuit is actually a ring, not a radial -
so the second wire could be the other part of the ring (maybe returning
from your back bedroom). If this is the case, you'd find that disconnecting
either wire alone would still leave all the sockets on the circuit live...
*and* the disconnected end live too - so don't be cavelier, and use a
multimeter to trace connections in preference to the mains supply!!
Or you might discover that the other wire supplies your rarely-used
immersion heater, in stark contravention of the Regs (an imm. heater
wants its *own* final circuit, not sharing with owt else, 'cos when it
switches on it pulls a Serious load for a Long Time, heating as it does
a rather greater volume of water than a kettle or a washing machine ;-)

Both the socket in the back bedroom powering the computer and the
socket powering the washine machine under the sink have 1 set of wires
connected to them - these are the two sockets that are connected to
the connector block by the sink. I've decided that its best to buy a
multimeter and find out once and for all whether the circuit is a
radial. Would anyone like to recommend a multimeter (the cheaper the
better!)

BTW - I will be protecting the outdoor socket by using a RCD adapter
and burying the cable in PVC conduit 45cm+ under ground.

When you say 'apapter', do you mean a plug-in jobbie made to go on the
end of a flex (not best practice for supplying a permanent circuit!), or
a nice accessory-box-mounted fused-and-RCD'd-spur-connection-unit thing
(the Right answer ;-)?

It's a 'Powercut Safety RCD plug', 30mA, 13amp fused. I was planning
to plug it in to a standard socket (located in a ex-coal shed which
joins on to the back of my kitchen).

For occasional use of kit outside, I think tapping into the 30A ring is
quite acceptable, given the constraints on your existing CU. At some
point, depending on time, resources, and other changes you want to make
to the installation, a larger (more ways) CU may figure in your plans, but
it seems like a 'nice to have' rather than a 'must do' on the information
you've given so far.

I agree - I think I will use the 30 amp ring - it should be ok to wire
up too because we just have floorboards in the kitchen!