There should be no difficulty in obtaining a test. You depart on the 29th so depending on departure time can get tested on the 27th, 28th or possible the 29th. Plenty of testing services available and I bet there will be one or more fairly close to you that is affordable too.We are due to travel to France on 29th December.
With the difficulty of obtaining a test and proving that it is negative, over the Christmas period, we have decided to postpone our trip.
Trusting a private company to respond in a timely fashion over the Christmas holidays is enough for us to back out. We will now spend our money on a tour in the U.K. saving our France/Spain trip until the spring when we hope the situation will be more settled.