I'm with Andy here...
And Kev, you know I wear Base Layers. Have done under my leathers for about 3ish years. I've always had them as I used to wear them for running and cycling. I also use Compression shorts/Long Johns too.
I have Helly Hansen, EDZ, MiiFit, NikePro, CCC underlayers and have to be honest...theres not much in it. But the price is pretty different. Those NikePro and Under Armour ones can be like 40 quid. Ouch.
All I would say, is now have two types (same make). Hot and Cold weather ones. Brilliant. When I rode to you and back last year, used the Cold Weather ones. Keep the chill off, even in summer over a long ride. But for short trips (day tours etc) I used the Warm weather one. Helped slightly last year in that fecking heat.
Bear in mind, anything lycra'ish based. Unless your Arnie or Lawrence Dalliglio you look like a fat budda. Ha ha. So you need to breathe in when you take you top off. I luckily have perfected the art of keeping my tummy in after some very unflattering photos' a few years back.