I very much doubt it. I've been a customer at Sandys for about 7 years and I think the reason the GMP tolerate it is because it is whiter than white in all other aspects of it's business. Apart from the obvious law it is breaking it takes a very tough stance on drugs, money laundering, trafficing, even smoking laws and fire regulations.
Going by the posts on another site the parlours that have been shut down (Hayleys, Toucan, WFB etc) have had other dodgy dealings going on which has brought them to the attention of the cops.
Like you, I very much doubt that corruption is involved. Police corruption isn't as rife as it was in the 60's/70's/80's. The decision to allow Sandy's to continue to operate is made at a very senior level within the police. Sandy's is allowed to operate because it suits GMP to allow it to operate (for the time being anyway).
Parlours are often raided within the GMP area although I've noticed that on another site are usually quick to explain the reasons why, and to make the point that it's business as usual elsewhere, and that there's been no hardening in the attitude of GMP.
The fact is that if you're running a parlour then you're:
1. Keeping a brothel used for prostitution (section 33A SOA 1956).
2. Controlling prostitution for gain (section 53 SOA 2003).
3. Money laundering (sections 327 etc POCA 2002).
4. (At a push) possibly trafficking within the UK for sexual exploitation ( section 58 SOA 2003) if you're facilitating the movement/travel of girls to work.
Like James, I doubt that Sandy's are "whiter than white".