The new £20 note

Why not take euros?

just use an ATM in Tenerife

Always get a better rate in Tenerife.

I think they should postpone the introduction of the new notes until April in fairness to bigmo.

Get yourself over to La Gomera

As the OP sounds like a regular visitor
Perhaps he should try using some OLD
peseauts

Thinly veiled went on Take Me Out and didn't get a black out.

I always take forged £20's when I go to Spain, they spend just as good as real ones.

Take Euros and a credit card. I'd be more worried about any last-minute changes to documentation/insurance requirements after Brexit (part one) especially if our continental chums cut up rough about Gibraltar or if President Macron notices the new £20 note includes Turner's painting The Fighting Temeraire, showing the scrapping of a ship that gave Napoleon's boys a damn good seeing to at Trafalgar.

For any chit-chatterers who have not "had enough of experts" ((c) Michael Gove) here is a National Gallery boffin talking for 25 minutes about just that one painting:
https://www.youtube.com/watch?v=8O-fna8HrWw

Erm, where were we? Ah yes, £20 notes. The old paper notes won't be withdrawn from circulation for at least six months after the new note is introduced, so if you are that worried about spending new notes abroad, take the old ones instead.