obtain an app from the Spanish IOS apps store and install it on my UK ID phone

16Mb Apple phone 5 with IOS 9.3

Having a problem of downloading to UK reg phone a commercial Diabetes Management application from a Spanish IOS app store. The spanish use a different measuring system than UK and I need the Spanish version.

I have created a Spanish ID and downloaded the application to it, but I need to be logged into the Spanish ID to use it, which is inconvienient

How can I transfer it onto the UK ID?