Applying for fresh tourist visa while current is still valid

[gre.bm691]

Hi all
Can someone please help?
My parents current hold 2-yr UK visit visa which is due to expire in June. The way I am planning their next trip, it is likely that they will be here in June. As the visa expires then, I want to apply for the 5/10yr visa now so that we avoid the hassle approaching visa expiration date.

Can I apply for a fresh visa while the current tourist visa is still valid? I am told that there are some countries that have restrictions around such applications.

[Frontier Mole]

This was recently debated - the answer is that you can apply while holding a current visit visa.

[AmazonianX]

Go ahead and apply, no such restrictions is known for UK.