Few quarries are set up (or have the correct geology) to produce large size rock armour and coastal defence material: its a specialist product for a specialist and intermittent market.
When the A55 expressway was built along the north Wales coast in the 1980s, most of the large armour came from Norway as most of the more local quarries either had inappropriate geology (e.g. joints too closely spaced to permit really large blocks to be consistently produced) or were not set up to produce such large products and simply declined to tender.
With a clutch of similar lagoon projects proposed for the region, it would make sense for the developer to acquire their own sources of stone.
Up-thread a total requirement of 200mt was mentioned for the Swansea lagoon. The initial scoping documents in 2012 proposed a 9.5km long impoundment barrier. Combining these figures suggests 2,100 tons per linear metre on average. Granite is 2.65 tons/m3. This suggests an average cross-section of the barrier of 800m3 (it'll vary considerably along its length depending on water and sediment depth) which seems in the right ball-park for a say 20m wide top, with say 30 deg. side-slopes, in the relatively shallow water of Swansea Bay which however has deep soft sediments.
The articles linked-to up-thread state that proposals are for Dean Quarry to produce 1.2mt a year. Comparing this to the suggested 200mt required for the Swansea lagoon alone, suggests that the greater proportion of stone will be sourced from elsewhere for the clutch of proposed lagoons.
Glensanda lists armour among its products; it might be also be a viable supplier; its stated capacity is "in excess of 9 million tons" according to its website but this includes servicing existing customers - though these are presumably reduced at present due to the downturn. I seem to recall some Northern Irish quarries were in the armour market also. There are presumably some other UK quarries that can also supply this class of material. Nevertheless, if a clutch of lagoons proceed more or less simultaneously then the total amount of stone needed seems likely to necessitate a significant proportion being imported.
The Swansea lagoon may have, I would guess, around a four or five year construction period - the developers will wish to see it completed and generating income as soon as possible. Within this, the period during which rock is emplaced will probably be no more than two or three seasons (winter weather will limit both working on site as well as transport). Working from the figures up-thread, this suggests deliveries of 60 to 100mt a year - double handling will be avoided so there will be no stockpiling; rock will be dumped directly in it final position. Delivery and emplacement of rock in the high tidal range of the Bristol Channel may be a significant limiting factor in the construction schedule.
If the Swansea lagoon is a financial success then construction the other lagoons will follow, with elements of the construction overlapping. Potentially more than one lagoon may be constructed simultaneously. I haven't looked at their various sizes but if their rock requirements are broadly similar to the Swansea one, then in around a decade's time we may be seeing significantly more than 100mt of rock a year being needed. Sourcing and delivering such amounts in short time scales may be challenging, especially if by that time an economic upturn has occurred and there are other demands for stone, especially armour.
Overall, Dean Quarry's contribution to the proposed lagoons looks likely to be modest in proportion to the total amounts of stone needed, even if Dean's production rises to a figure somewhat above the 1.2mt a year currently talked about.
OS maps don't suggest enormous scope for expansion at Dean - it is difficult to visualise a full-size coastal super-quarry there. Perhaps it may develop into a 1-2mt a year quarry for a decade or two whilst the Bristol Channel lagoons are being constructed.