I don't know of any specific distance specified anywhere. General requirements will mean you'll need to take account of ambient temperature though - which will almost certainly be higher for a floor with any kind of heating system incorporated into it. You'll have to talk to the UFH designer about actual temperatures - it'll vary depending on a number of factors including the maximum required heat output per sq m (which in turn depends on the building size/shape and how well it's thermally insulated), the thermal resistance of floor coverings and so on.  While BS 7671 generally assumes (e.g. for cables) an ambient temperature of 30 degrees and calls for de-rating above that, there will also be the specific accessories to consider. BS 1363 for example IIRC says max 25 degrees average over 24 hours, which might be more difficult to work around.

     - Andy.

Minimum distance in which direction please? Above or to the side?

If a floor box is surrounded by the heating, it is likely to be at that temperature. 30 - 40 deg C? Much higher and you wouldn't be able to walk on it with bare feet.

New build? 450mm minimum from floor level.