When you say 19-21mm, I’m assuming you mean internal width?
If that’s the case, then you’ve got a lot of choice when it comes to rims, and loads of choice on hubs too. You certainly aren’t restricted to Shimano centrelock hubs if that was your worry.
Depends on your budget, but you won’t go far wrong with a Hope Pro2 front hub built onto either a Stans Flow or a Mavic EN521. Both these rims are circa 21mm internal width, you could go for a Stans Crest instead which is slightly narrower and lighter, but still reasonably wide also, but not as strong as the first two. Hope Pro2 prebuilt wheels aren’t that expensive when you consider you can pick up a front wheel on its own for a little over £100, and the hub price is £55, the rim is £60 in the case of the Stans, and the spokes are probably £20 worth, so you’re actually saving money buying a fully built wheel!
Looking on Superstar’s website, one of their own brand Switch Front hubs on their own brand rim, built with double butted spokes, comes to £90 anyway. Personally, I’d pay the extra for a Hope wheel myself.
Or there’s always ebay for a 2nd hand bargain possibly…
http://www.ebay.co.uk/itm/Mavic-XM321-Rim-Sun-Ringle-Abbah-Hub-20mm-26-MTB-Front-Wheel-/160779197414?pt=UK_sportsleisure_cycling_bikeparts_SR&hash=item256f2fdbe6
http://www.ebay.co.uk/itm/DT-Swiss-FR2350-front-wheel-20mm-red-/300695160800?pt=UK_sportsleisure_cycling_bikeparts_SR&hash=item4602d407e0
http://www.ebay.co.uk/itm/Mavic-819-Front-Wheel-Hope-Bulb-20mm-Axel-/130678915526?pt=UK_sportsleisure_cycling_bikeparts_SR&hash=item1e6d1201c6
etc.