Yes, in my case the existing windows were timber, but acted as part of the structure i.e strong enough to act as a lintel. As per
@stuart45, upvc is not designed to be structural in any way re-enforced or not. So removing the structural timber windows and replacing with UPVC without adding a lintel is when problems will start, although it may take a few years to become apparent.
Hence, ensuring a correct lintel is specified.
Nearly every builder (bar one, who was very expensive) I have spoken to and every UPVC company I have spoken to have specified an angle iron (specifically meaning an angle iron and not a "catch-all" for lintel). The response being that it is a good enough solution for nearly all the work they have ever done and anything more is overkill.
It seems that this is the de-facto choice, unless you question it.....like I have. I even checked with a local architect who said while an angle iron would "do the job", it was certainly not the best nor most appropriate solution, in his view.
It seems that the window companies do not like using proper lintels for what ever reason, (extra cost which makes the sale less attractive? time?). Trying to get a builder to just install a proper lintel is also difficult as it seems, for most, the job is too small in the current climate.