I am a flooring contractor, hopefuly you find the following information helpful.
Engineered boards are by far the best for stability when it comes to UFH or installing in a newly built property.
The probem you usually get with a newly built property is the moisture levels in the property are to high in the concrete floor and concrete has a very slow drying time, engineered boards can usually be installed on a concrete floor with moisture levels of up to 6%. Solid floors need the levels to be less than 4%.
Semi solid floors (3-4mm top layer on a 2 ply softwood backing) are about the same as a solid floor when it comes to moisture levels but specifications vary a lot from different manufacturers so you will need to check. They are usually cheaper but to often give problems with expansion especially with the very cheap ones.
Solid floors should never be installed as a floating floor, if anyone advises you differently do not use them (I can show some real disasters people have had from doing this). Solid floors can be installed over under floor heating but it will be a lot more expensive because there is a lot of preparation to be done first. If you are installing a solid floor over under floor heating make sure your contractor will give a written guarentee and get a VAT receipt off him as proof he did the job before you make the final payment.
The best way for installing a solid wood floor is to sheet the room out in 15mm plywood and nail/staple the floor to the plywood. The reason for this is if you get any damage in the future (ie. leaking radiator pipe or water coming down from bathroom upstairs) it is very easy to replace a small section of the floor with minimal disruption to your house. Also it will be much quieter under foot.
You can also bond it directly to the concrete but make sure the concrete is sealed first with a suitable primer and a flexable adhesive is used. Good manufacturers of these products are Sika, F. Ball and Bona Kemi, we use these products all the time and never get problems.
With any flooring installation your fitter should check the moisture levels of the Sub-floor the floor being fitted and the relative humidity levels of the room where it is being installed. You should always ask for a VAT receipt as proof the job was done and a written guarentee. The fitter should give you details of what is included in the price - The floor itself, cutting doors, new skirtings/scotia, underlay/glue/plywood, door trims/reducers, all labour for the job, VAT, taking away the rubbish, all of this is very important as I hear about people being ripped off by someone sounding like they had got the cheapest price but when all the extras were added on it worked out a lot more expensive and the home owner was left with a pile of rubbish to deal with