Christmas is gone now so maybe they will go soon too? I've seen Reindeer at ZSL in the past, they used to be in an enclosure roughly opposite the outside of the Camel part of the Casson Pavilion, up against the Zoo boundary fence. In summer they tend to look motheaten and miserable, no-one will give them a second glance in the summer season.
There was also a small pen with a stable on the lawns before Christmas next to a Santa's grotto.
I think this enclosure makes nice use of a forgotten corner but it doesn't look very permanent and I wonder if they have plans for this part of the zoo.