what would United Kingdom do if it was WW3?

Any future war will not be with Russia.
We have lived under the M.A.D. threat for long enough to know how far to push each other.

War with China?
I don't know but i'm sure it would result in a united Europe / American / Oz force.
Again, I suspect the M.A.D. threat will negate the use of nukes.

The most likeliest scenario is confrontation with fundamental Islam states and groups.

What would the UK do?
I don't know.
At present public opinion is vehemently against any conflict with Iran.
This could easily change depending on developments and propoganda.

Current events in Pakistan could have a major bearing on near future events.

War must be the last resort in any scenario and every avenue for peaceful and diplomatic settlements must be exhausted.

But one thing is certain; if it happens then Britain will not be found wanting.