Many thanks for your patience. This is unlikely to be garden leave, which happens when you are serving your notice period, so it will most likely be period of suspension.
Being placed on suspension is not an automatic assumption of guilt and does not amount to disciplinary action. Whilst it can lead to disciplinary action, it is primarily there to be used as a precautionary measure whilst an employer investigates any serious allegations against the employee. Reasons for suspending could be in the case of gross misconduct, breakdown of relationship, risk to an employer's property, their clients or other employees, to preserve evidence or ensure it is not tampered with, avoid potential witnesses being pressured or intimidated, etc.
The period of suspension should be as short as possible and kept under regular review. During that period the employer should conduct a reasonable investigation into the allegations against the employee. If the investigation gathers enough evidence to justify taking disciplinary action, the employee has the right to be informed in advance of the allegations and evidence to be used against them and be given the opportunity to prepare to defend themselves at the forthcoming hearing.
In terms of issuing you with a warning, that is possible if you have potentially affected the relationship with their client. I must point out that the law allows employers to even dismiss someone if a client of theirs no longer wants them working at their sites and there is no other work available, so a warning will not necessarily be that bad considering that. Of course, you can still challenge the employer at any forthcoming disciplinary hearing and explain your side of the story, hoping that the employer takes that into consideration.
One more observation is that the employer should not have decided the outcome before you have had the chance to go through the formal process and it suggests the outcome was pre-determined, which would allow you to challenge it on these grounds as well.
Does this answer your query?