open google maps to bus directions

From MapKiWiki:

dirflg Route type:

• dirflg=h - Switches on "Avoid Highways" route finding mode.
• dirflg=t - Switches on "Avoid Tolls" route finding mode.
• dirflg=r - Switches on "Public Transit" - only works in some areas.
• dirflg=w - Switches to walking directions - still in beta.
• dirflg=d - Switches to driving directions.

So your URL:

NSString* urlString = @"http://maps.google.com/maps?saddr=London+UK&daddr=Birmingham+UK";

Becomes:

NSString* urlString = @"http://maps.google.com/maps?saddr=London+UK&daddr=Birmingham+UK&dirflg=r";

N.B. The data supporting public transit route types are not always available. I tried the area you used in your sample (London, Birmingham) and found it was not supported.