///mrs chappie; I know nothing on the subject I admit, but if the rim is one-piece the original lens has to have been fitted in there somehow, I would suggest by heating and shrinkage, like the tyre on a wooden cartwheel.///
Cheap metal welded rims that cannot be opened tend have the lens bonded in at the point of manufacture, or the split rim is riveted together at the point of manufacture.
These are the "ready-made reader" type of glasses - glasses bought "off the shelf" at low cost. They are manufactured as a disposable product (basically with cheap and shoddy frame). It is not worth an optician's time and effort to attempt to reglaze these frames, as more often than not they break.
Ratter, those frames in your link look reglazeable (and should be, if they are being sold as such). OH charges a tenner for this service (edging the lens to size, and fitting) plus, of course, the cost of the lens.