See extract below of HMRC manuals.
Effectively if you have really valuable information that would justify a monetary reward - as opposed the Gordon Brown's everlasting appreciation for the duration of the parliament - then you would be best advised to approach one of the HMRC Specialist Enquiry offices. S.32 Inland Revenue Regulation Act 1890, gives the Board of HMRC authority to make rewards. However, the case of Riach v Lord Advocate  18TC18, confirmed that this merely authorises the Board in their discretion to reward an informer and that it would be beyond the BoardÂ’s legal powers to enter into any agreement or contract in the matter. An informer cannot therefore acquire a legally enforceable right to a reward.
If an informer tries to bargain for a reward as a condition for giving information you should tell him or her that the Board will not commit itself to making a reward, but that the matter must wait for consideration by the Board until after any enquiry has been made. If necessary, send a letter in the following terms
Â‘I have to inform you that, with regard to a case of alleged evasion of taxation, in the event of information furnished by you being found by the Board of HMRC to be of value, a reward might be granted. But, the question whether any reward is to be given and, if so, the amount of such reward, must obviously depend upon the value of your services in leading to the recovery of duty. It must await the consideration of the Board after the matter has been investigated by their Officers. I would add that any information you wish to give will be treated as confidential and that your name will not be divulged without your consent except where compelled by law.Â’
unfortunately you won't be able to get a commitment to any fnder's fee until the money is in the kitty