Hi,
I'll keep this frank as loft conversions,velux and dormer's are like a middle name to me, so to speak. So without too much jargon I'll keep it easy.
1. Velux brand is the name given to the company that makes a flat fitting type window which open either by sliding and/or angled openers. You may see them on some roof's. These are added for light without the need for creating additional roof alterations to add head space, for example a loft conversion; loft space above the house is converted to living space. For light one could use velux type windows. This additional space does add value to your property.
2. With a dormer you do away with the need for velux type windows entirely and can add more open space and headroom with plenty of natural light. The roof and the floor are supported/opened up or even raised to become more like a terraced type attic in appearance. These are a little more costly then a velux type conversion but are well worth it as they add more to the property value then just a velux type conversion.
3. UK Management Builders can convert a loft to a dormer or velux style. Most competent builders would be able to do the same. However UKM Builders also advises you speak to an architect about design/space/property value and pop into the Local Authority planning office and just ask them if you need permission as they will advise you of fees and cost/regulations etc.
Hope the above helps.
Kind regards
UK Management Builders LTD