Normally you will only find problems with really slippery slipways at low tide -- try to stick to half tide or better and you will find that many slipways aren't slippery at all.
If things look really dodgy, do not put your tow vehicle in danger. Keep on the hard, dry section, and use a long rope to lower your trailer down the ramp. This also applies to shallow slipways where you might otherwise be tempted the temptation may be to back your car up to the sills just to get the boat off the trailer!
For good places to launch, have a look at www.boatlaunch.co.uk
-- if you know a good place that isn't listed, then let the webmaster know!