Just had a look on the micromark site and read the instructions -
Clicky
It does say rather clearly 'do not wire into the lighting circuit - use a switched fused spur' but fails to either say why or state what fuse they think you should use.
I am guessing they are perhaps saying it because it is designed for outside use and therefore more likely to be exposed to damp etc. (though that by itself isn't a sensible reason to use a fused spur).
However, in the absence of them stating a fuse requirement, I can see no reason at all why you could not use as you wish (directly into the round 3 pin socket). Looking at the product, a 5A fuse should certainly be more than adequate and it should never draw more than a fraction of an amp anyway.
Having said that, if you go against the manufacturers express instructions they will likely not provide any warranty or support if something goes wrong.
It may be worth ringing their 'technical helpline' and asking about using it indoors and seeing what they say.
You could comply with their instructions by replacing the light switch with a fused spur but personally I don't see how that would make anything more safe.
Regarding a dimmer, you could replace the light switch with one, but personally I've found that dimming GU10s can shorten their life expectancy significantly. If you do it, you need to get a dimmer that is rated for halogen loads, or 'derate' the stated normal load by 50% - in your case with two lights a standard 400W would suffice (250W would probably do but a little tight), or one that is rated to accept 140W halogen load.
Gavin
p.s. Have you discovered what the fuse is on that circuit - if its an old install I'm slightly worried that its a very old power socket we are talking about and not wired to the lighting circuit at all!
