Brexit won't have a bearing and Batflu shouldn't either, the islands need the revenue.
Brexit shouldn't affect the 1/2 week a year bucket and spade brigade (barring passport queues), but may affect long stayers depending on your residency rights. I was hoping to do a season (December to April) in a ski resort before I retire, now I can't without a long stay visa. You can't go to any place in Schengen for more than 90 days in any rolling 180 day period. Overstay and you're potentially banned from the whole of Schengen for 2 years. There are also additional health insurance and financial requirements which, although rarely checked, could trip you up.
Batflu is a two way thing - even if the Spanish do let you in, there is no guarantee those lovely HMG people won't lock you away in a Gatwick hotel for 2 weeks on your return, adding £1750 per person to the bill. That's what is going to scare off most holiday makers. You can abort your holiday any time up until you set foot on the outbound plane, but once you're in 'foreignland' you're at the mercy of what HMG do as to whether you can get back in again.