it's not the max. wattage that's the problem. it's the min. if you have a load of only 20watts and the dimmer min. is 50watts, that's why. also the dimmer usually needs to be trailing edge.
If you go for a V-Pro silent trailing edge intelligent LED dimmer it will cope with up to 300w which is much more than you need. They are also designed and made in the UK which to my mind is a big + point. A 1 gang dimmer will be around £25 +VAT. LED's are very difficult to dim right down to zero without flickering (although you see it as an LED lamp it may in reality be 25 or so LED chips all in the same housing and any slight variability across the 25 may cause individuals to flicker on and off at low levels) To get round this many dimmers have an adjustable minimum which you can tweak to give you low but stable illumination before pressing the switch to turn them off completely.
If the dimmer module does not have a connection to neutral (which I don't believe the Varilights do), because they require power to operate the touch electronics, it's highly likely the LEDs will continue to glow even when they are "off" due to the current flow through the dimmer.
I know because I've tried a V-Pro touch dimmer and had this exact issue.
Varilight (and others) also sell snubbers that are designed to stop this happening. Last time I looked they were about £25 I think. In the end I decided I'd simply ditch the touch idea and go for a normal rotary dimmer.
Had same issue about a year ago and trying a snubber didn't work; was working with a guy much more experienced than me (electrically) and (think) he came up with some way of switching the neutral as well. Can't for the life of me remember how he did it but am working with him this weekend so will ask. Pretty sure it was only achievable because they had 2 dimmer modules/remotes though
One thing not to do is your suggestion of mixing in one halogen on the same circuit, as when it blows the resultant voltage spike will most likely knacker all the expensive LED lamps.
I agree with the others: you need to buy an LED-specific dimmer module.
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