I have resaerched flannel wasitcoats,for some months now, trakcing down every reference to it I can find in all my books on 18th and 19th cnetury costume and also in the library.

It has been alleged that the wearing of flannel waistcoat was an indication not of any aged habit or of infrimity in the weret,but of manly activity, especailly sporting activites.

So to many "manly" activities I have gone, to see if flannel waistcoas were worn,and one could attribute to the wearer of such a garment boldness and bravery.

The only soorting refercne I can find which specifically advises the use of flannel waistcoats by men ,is in the most authoritive book on the sujbect of sporting clothes, English Costume for Sports and Outdoor Recreation(1969) by Phillis Cunnington and Allan Mansfield.

This book refers only to flannel waistcoats being advised for use by anglers to keep them warm on cold and damp river banks.

In another authoritve book The Handbook of English Costume in the 18th century b by C.Willett Cunnington and Phillis Cunnington I found this specific refernece to flannel waistcoats:

Under-Waistcoats.

These, designed to be seen came into fashion in the 1790's.

They had shawl or occasionally stand collars, appearing above the turned back lapels of the over-wasitcoat; and were square-cut and closed with two or three buttons.

The visible portion might be of brihgt silk, the rest a quieter material. The under wasitcoat was often considerably shorter than the over waistcoat, perhaps not more than some four inches deep.

Under wasitcoats desinged for warmth only and sometimes called "camisols" were in the nature of an undergarment; usually of flannel or similar material and sometimes made with sleeves, there were in occasional use throughout the second half of the century

I cannot be any more postive than that, and I think we have to accept JA's own assessment of the wearing of a flannel wistcoat, that it indicates nothing more manly than being sensibly concerned for one's health: that it is a "constitutional safeguard".( see seventh paragraph from the nned of the last chapter of Sense and sensibiltiy)

As for the use of flannels in Bath, they were certainly meant ot keep one warm. Here is a link to an old thread about the use of flannel while bathing in the baths at Bath ;-)