The idea that they use animals cos they're cheap is an absolutely ridiculous notion. each one of these animals is specially bred and costs thousands of pounds, and that's before the lab has paid for their licence (another few thousand), housing, food, medical care etc. alternatives are massively cheaper but currently there just isn't an alternative that is as precise and accurate.
Beagles are used for several reasons 1. historically that's what they used and because they are bred specifically to breed out diseases etc starting new lines of different breeds would take decades 2. generally they do well in packs which is the environment they live 3. using lots of different breeds may result in different results 4. they're the right size to be easy to use.
There are strict rules on using dogs and they can only be used when there is no other animal suitable (with the exception of primates)
FYI i've been in that facility and the dogs are actually well cared for and loved by the carers. they are most definitely not "stacked". I do not, nor have i ever, worked for the facility though
If you want to know more then the website understanding animal research is absolutely fabulous