UPVC porch, can it be easily re-built in brick?

Yes it can be down it would need be cavity wall just dig a test hole down side foundation to underside concrete to see if it deep enough to build off

Firstly we would need to inspect the foundation to see that it’s suitable to take the load and meet regulations, minimum of 1m deep in clay and slightly less if in chalk or gravel. If a new foundation is installed the walls can be constructed with concrete blocks upto damp course internally and ground level externally. Cavity wall to be 100mm internal , 100mm external and a 100mm cavity. Damp course can be laid lapping onto the existing house damp course and tying up internally with the floor Dpm. Walls can then be costructed with 100mm insulation blocks internally with wall ties every 2 course of blocks and every course up the sides of openings. Insulation can then be fitted to the blocks using retaining discs (rigid insulation part fill or rock wool full fill). The outer leaf can now be constructed with 100mm facing .

I agree it's easy to covert into cavity walling. Regarding footing's like mentioned test holes to be dug to firstly inspect the depth of foundation, but also quality of the concrete and more importantly the ground quality it's bearing on. Timber frame with brick external skin would be your best bet as would reduce the load.

All depends on weather you wanted to get building control involved To get it signed off at end off job Test hole would have to be dug to see footings were there Some times just as easy to knock down and start again As insulation should be in concrete floor

If you are going from a single skin to cavity you will need to check that you have sufficient width of foundations. A single skin can be built on as narrow as150 to 200mm your new cavity requires at least 400mm.