The vast majority of sash windows can be fully refurbished and will outlast a modern replacement. Georgian and Victorian sash windows have been manufactured from slow grown pine which is a far stronger, more durable timber. When your sash windows are fully refurbished they will last another 100 years – as long as the exterior paintwork is maintained.
For grade A listed buildings the answer will generally be no, but for most other properties it is not a problem. A phone call to the local planning authority will clarify this.
Yes, modern materials are used to upgrade the traditional sash window, improving overall heat loss.