To what extent is remuneration under a PPL enforced in the UK?
The rules are as follows:


  
  Privileges and limitations
  
  
  A holder of a PPL may, for compensation or hire, act as a crewmember of an aircraft in connection with any business or
  employment if—
  
  
  
  a. the flight is only incidental to that business or employment; and
  b. the aircraft does not carry passengers or property for compensation
  or hire.


"Compensation or hire" means income, not expenses. You can always be reimbursed for expenses such as fuels. Only income is forbidden under certain circumstances (see below).

Concerning getting income, you can be paid income if both of conditions (a) and (b) above true. That means your business cannot be anything to do with flight. If your company is Boeing or any air-travel related business, you cannot receive ANY income (even your salary) for piloting such a flight.

The aircraft cannot be used to carry paying passengers or cargo.

So, basically what this means is that if you fly anybody from your company, they can pay only the expenses of the aircraft and nothing else. You can be paid your salary, only if flying is not your job in any way. As long as you have a regular job and it is fully incidental to flying, then you can fly clients and fellow employees and receive expense reimbursement only. If you are flying clients, they cannot in any way be paying money for the flight other than direct expenses like fuel.

The usual consequence of violating these regulations is that your license to fly can be suspended or revoked. Technically speaking, flying for hire without a license is a minor criminal offense (see below), however prosecutions in the past have only been given out for egregious violations (like working for a charter company without a commercial license).

The way these regulations are enforced is the following:

(1) If you are flying passengers or cargo for hire, and somebody rats you out, complains to the CAA, or it otherwise comes to their attention, then they will investigate and revoke your license if they determine you are flying for hire.

(2) The CAA has inspectors and they may do surprise inspections of aircraft. If you start flying passengers regularly, you may get inspected. If the inspector finds you are flying a bunch of passengers with a PPL, they will closely question your passengers about what money those people are paying. If the inspector determines they are paying money other than incidental expenses to be flown, you will likely have your PPL license revoked.

EXAMPLE:

You are flying 3 colleagues to a business meeting. A CAA inspector directs you to park on the ramp and inspects you and finds out you have a PPL. He will then question your passengers. How much are you paying for this flight? Colleagues answer:

(1) "nothing". You are ok.

(2) "18 pounds". You are probably ok.

(3) "200 pounds". You will lose your license.

If he see cargo on the aircraft, then there are two possibilities:

(1) He determines that the cargo is incidental to the flight and it belongs to you. (Like scientific equipment and you are a professor flying to an archeological site). You are ok.

(4) He sees cargo on the aircraft that does NOT belong to you (like 8 cases of computer chips being delivered to an electronics company). You will lose your license because you are flying cargo for hire.

Criminal Issues

Note that technically, flying for hire is a minor criminal offense in the UK and can be punished with a fine up to 2500 pounds in a magistrate court. However, it is very rare for this kind of offense to be criminally prosecuted. In the linked news story it was found there was only a single prosecution over the course of a year, and in that case it was a pilot working for a charter company and doing many commercial passenger flights, which was a blatant violation of the rules.