In Top vs Bottom mode, computer players will respond to allies who are under attack, but in UMS mode, the AI will not provide any support to allies at all. Is there any way to make the AI send units to support allies like in TVB

In UMS, computers will only run their AI scripts if explicitly told to with triggers. If you run one of those scripts then they should behave more normally.

If you're running unmodded, there's a script that you should run for the AI player that should be defended, and in theory the allies of that player might provide help.
It's called "Brood Wars Zerg 1 - Town F". It contains the "help_iftrouble()" command that should help.

I don't think help_iftrouble will work for human players.

If you're trying to coerce the AI to do things in UMS, if it's not just a melee style UMS you can force the AI to defend certain areas by running scripts at different locations or making them build bases in the middle of nowhere by placing start locations there, each unique start location will be marked as a base location for the computer. If using EUDs or modding you can try placing hidden structures that the AI will "defend". You can also use a second scriptless computer player of the same colour to take control of some of their units to help you out with triggers.

To achieve this effect I'd generally spam ai value area higher around a players' base in UMS but that's not super useful for the original top vs bottom question