As a Cocktail Bartender myself I thought I would step in and shed some light on this - i've worked in two bars and both have, to an extent, used Funkin for their Cocktails. The first I worked in was fairly middle of the road and had fresh limes and lemons for use in all cocktails but Funkin and flavoured Vodkas for everything else. Considering the price was £4.25 for all cocktails and it often got very busy, this was acceptable, but their plans to introduce a pre mix Margarita Syrup did not go down well with the staff. The second bar was much more upmarket and was charging more for the drinks, here there was a much greater array of fresh fruit but Funkin was still present in case supplies were running low or at high volume periods.
Alton's cocktails are very similar in quality to the first bar I worked in, and to be fair when the OP said premix cocktails, I imagined the sort of Pina Colada Mix you get from Tesco. Funkin is a bit better than that as at least the spirits are mixed, but its still a lazy shortcut for a £6. My guess would be that Alton won't sell near as many Cocktails as a trad Cocktail bar, so they could end up buying in a variety of fresh fruit which doesn't get used and would then perish two days later. In the Merlin World of profit dictating any decision making, this just wouldn't work.
Also to be fair on Alton, whilst £6 for a drink is expensive, as far as a Cocktail in a hotel bar is concerned, its really not that bad. I payed £7.50 for a Margarita in a 4* London Hotel and it was made using Premix stuff. safe to say I wasn't best pleased!