For Part M requirements See Table 2 on page 28...

http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADM_2004.pdf

The min width required depends if a wheel chair can approach the door straight on or if it has to "turn in" due to the positioning of a ramp. Assuming a straight approach is possible you need a clear opening of > 800 = 31.5". This means you need a door that is at least 34" wide.  

In your case you would go for a 36" or 914mm wide door which needs a rough opening around 38" (eg 1" either side). You have 39" but that should be ok. May need a bead/moulding around the door to avoid a wider than normal  mastic gap.

Regarding the height... Is there a wheel chair friendly sill fitted already or do you need a door frame that includes one?

Nearest standard door height would be  6’10’/82"/2083mm leaving one inch at the top for the frame at the top... but I think 36" wide doors are normally 7 foot/84"/2134 tall.  Check the door can be cut down 2" before you buy.  

Fuller list of standard sizes here...

http://www.door-warehouse.co.uk/size_convert.htm

but not all combinations of width and height are available.

Normal proceedure to calculate the rough opening size is to add 2" to the width and 1" to the height (above the cill). The frame is 3/4" or 22 mm leaving a bit for packing to get it square. On an internal door the architrave covers the gap. External doors generally get mastic and or beading.

I suspect engineered oak would be more stable (we went for engineered oak flooring over UFH) but I have these solid double doors fitted two years ago and they have been fine...