United...What is the significance of the suffix 'United' used by so many British football teams?

United is used when a club is created after two former clubs and combined or united.

MANCHESTER UNITED:

Here are two examples: After nearing bankruptcy in 1902, the club was taken over by J H Davies who changed its name to Manchester United, chosen over Manchester Central and Manchester Celtic.

NEWCASTLE UNITED:

The club formed in 1892 after the merger of two local clubs, Newcastle East End and Newcastle West End, and has since played at St James' Park, the old Newcastle West End ground in the city.

Like others have said just symbolising unity within the team and togetherness at the club so it has become used by many.

I have a feeling EK is right and that Sheffield United where the first team to use it as I think there one of if not the oldest club in football.

Wikipedia says the first team to use it was Hanover United FC

some of them refer to the "uniting" of smaller teams in the area to form one stronger team to represent the area better. or the "uniting" of clubs that compete in different sports but under the same name

Possibly Sheffield United, but don't quote me on it.

It's meant to signify a unity and togetherness of the people of the area.