If you have done a good job of insulating the loft floor you don't need to do the rafters...probably better that you don't unless you want it to be some kind of living space...
If you do decide to do it be very careful that you ventilate properly you need to leave a good 50 mm gap between the insulation and the tiles and make sure that air can flow behind them -
wickes used to do a good leaflet about it - not sure if they still do or not...
basically air must be able to come in behind the insulation at the roofline and flow all the way round to the oppostie roof line.
You might need to increase the depth of the rafters and even replace some tiles with ventilated ones...
To insulate them you can use loft insulation roll fixed up with netting or use something like celotex or polystrene boards - the latter 2 won't need covering .You can cover with plasterboard...vapour backed stuff...
Lots of people have problems with condensation in the loft - so make sure even if you only do the floor that air can enter above the insulation...
I'm not an expert but due to the funny layout of my house I
decided to have my rafters insulated instead of the loft floor ...it was so complicated I decided to get a company in to do it for me and they did a really bad job and on advice of an architect, builder and structural engineer it all had to come down...
I then tried to insulate just one area of the rafters - with netted up loft roll but leaving a generous gap behind -after a year when i took some down (by pure chance) I thought my roof was leaking - in one patch approx 1ft by 1ft the wood was soaking wet with signs of the start of mould growth (my roof is wood lined under the tiles) - only to find my roof was sound but the insulation had been pushed in filling the gap ....I found the same thing in another patch when I gave up and took the whole lot down... I really wouldn't do it unless it was essential...
