Just seen a report on Granada Reports that Greater Manchester Police had to borrow police officers from other forces after the EDL turned up in Manchester due to not having enough of their own officers. It wasn't mentioned where the other police were borrowed from but they were wearing Welsh police uniforms, which gives a clue.
As others have said it's pretty standard for anything like this or any major event. It's called "Mutual Aid". It takes advantage of having Public Order trained PSU/TSG units in Forces up and down the country, letting them do what they're specially trained to do and helping not to overstretch any single Force's resources too far.
IIRC, in many Forces the majority of PCs aren't Public Order trained, so Mutual Aid can provide necessary extra fully trained (known as Level 1) Public Order PCs that the host Force might not have. I don't know that this would be the case in Manchester though, as I believe GMP traditionally used to give all its PCs Public Order training and it's a very large Force.
Another benefit of Mutual Aid is that without being used outside their Force area, some Forces PSUs/Level 1 Public Order trained PCs wouldn't get much/any practical experience of large scale Public Order policing. So it keeps them battle ready.