Travelling by public transport

There is a distinct shortage of people in cars so the chance of me or Tom being able to pick you up from Santander is slim... (we've got vans so only one passenger seat).
For public transport, if anyone can get to Arriondas, Cangas de Onis, or Covadonga around the time Tom and I arrive (or before we walk up the hill) we'll happily shuttle people to Los Lagos.

Public transport options to get to these places:

From Santander:
Bus to Oviedo, change and get bus to Cangas de Onis. About 4hrs. The advantage is that bus goes from airport as I understand it and there are frequent buses
Train to Arriondas from Santander - just over 3.5 hours but need to get to train station from airport (two trains per day)
Bus to Llanes then change for a bus to Arriondas - just over 3 hours including waiting time (fairly frequent buses to Llanes, its the leg from Llanes to Arriondas which is less frequent)

From Oviedo
it is easier as there are direct buses to Cangas they run frequently and take 1hr 30mins. There are flights from London to Oviedo.

Timetables
All buses can be looked up here https://www.alsa.es/
Trains here https://www.renfe.com/es/en/suburban/cercanias-feve/timetables

If you are trying to look up options for booking a bus or train then beware that sometimes nothing comes up when you search too far into the future because tickets are not yet on sale. You will also have to break to journey into stages as the websites seem to struggle with connections

Taxis
There look to be taxis (suggest pre-booking) in and around Arriondas or other places which could get you the final leg of the journey if no one with a car is around.
https://www.taxisella.com/

and in Cangas going all the way to the lakes (but I guess they would pick someone up from Arriondas)
https://www.taxitur.es/cuanto-cuesta-subir-a-los-lagos-de-covadonga/