How would i change the T*'s to T5's on my rio 125?

How are those endcaps for your current T8s attached? If they are screwed in or attached by some other method that lends them to easily be removable, changing them out for T5s should not be particularly difficult. You may also be albe to add more bulbs if there is indeed room for them. You would need:
1) appropriate T5 ballast(s) -- the exact type and number will depend on the length of T5s you'll be using, total number of T5s you'd like to have, and/or how much independent control you'd like to have over the bulbs
2) T5 endcaps
3) T5 stand-offs / mounts
4) T5 bulbs
5) T5 reflectors -- important, as they are what makes T5 HO lighting as useful as it is
6) cooling fan(s) -- optional, but often a good idea; many people use computer fans on a DC transformer

You can certainly buy a retrofit kit that includes all the parts you need (except for the cooling fan(s) usually) -- some of which come prewired / preassembled -- or you can purchase parts separately. Which of those is easier or more cost effective might be different in England than it is in the US, so it'll be worth checking around some. A retrofit kit usually includes plenty of electrical wire to hook up the lights and a power cord for the ballast.