In FF's post 1026 he states
The Airport's financial reports are in the public domain. - using companies house filings. It made more than £11m profit in 2018 on a revenue of £30m or so - a good business - but since then has lost heavily:
So the 2018 profit was based on short haul regional flying and a smattering of charters.
The losses since then are down to Flybe and Covid. SOU has recovered slower than other airports due to the loss of BE, but the evidence seems to be there that those gaps are being filled by LM, EIR etc. The runway extension will benefit those flights and might also see an increase in charters and W flights like EZY's ski flights. There'll also be a contribution from new business park.
I don't see a LCC base as the be all and end all