Philippa generally uses Bottom Line made by Superior Threads which is a  #60 weight 2 ply polyester thread. Often she uses this top and bottom, but suggests you might also use Superior Threads Sew fine in the top if your machine doesn't like Bottom line in the top.

Polyester is lint free, stronger than cotton, and stretches rather than snaps (cotton snaps) so you can use a finer thread without the thread breaking, and without it adding a lot of bulk.

Philippa generally uses a 60/8 microtex needle (which is a very fine needle) for all her work - and she changes them often!