According to "Standards for Effluent Discharge Regulations General Notice No.44.of 2003 THE ENVIRONMENT PROTECTION ACT 2002 Regulations made by the Minister under sections 39 and 96 of the Environment Protection Act 2002", it states that the maximum permissible temperature for discharges into the aquatic environment is 40oC.

Also according to an environmental agency (United Utilities water/wastewater emergency) " Temperature  is limited by statute to a maximum of 43.3°C" (http://www.unitedutilities.com/documents/Trade_Effluent_Information_PACK.pdf)

I know these are not specific for UK, but I hope it helps your judgement

Likewise, I do not have the UK criteria.  They requirements should be in the UK water quality standards, statutes, or linked to their process for permitting of discharges.  If there are minor discrepancies, use the most confining temperature limit.  A lot would depend on river flow during withdrawal and return, as well as how many industries are doing the same, what is the net effect.  Permits to discharge would take into account the existing activities and river temperature data.  In many instances, the river minimum flow may be used in calculation the estimated worse scenario or set conditions when no more discharges are allowed.  There may be ways to limit thermal discharges, cooling ponds, recirculating, etc.  The need for limits may only be seasonal.  Although standards or guidelines are often offered, it is still best to evaluate the specific circumstances, define any boundaries or limits, potential effects to water quality and aquatic species, opportunities to mitigate by storing, recycling, cooling tower, seasonal, river temperature or flow limits, etc.  A requirement for monitoring and reporting, penalties for lack of compliance may be stated in regulation or determined for specific site.  Permit renewal may depend on compliance.  In instances of ample year round River flow, if the minimum River flow is 100 cu m/sec and withdrawal and return is only 0.3 cu m/ sec, then even discharging boiling water might be difficult to measure when assimilated.  The data and calculations make the analysis so discussions can begin and decisions can be made.  

If you are getting major discrepancies in information, and there are no clear regulations or process to follow, there may be problems within UK regulations, that no one but UK can resolve.  Careful consultant is going to avoid putting business and credentials on the line or be subject to lawsuit unless as stated, in evaluating the worse case scenario, the temperature increase is not measurable or the industry agrees to cover all liability.

CORMIX simulations indicated that

both discharge temperature and river

fe the plume extended from

shore. Plume distance from shore was

greatest at the lowest river flow tested.

As river flow increased, the

plume was more quickly turned

downstream and distance from shore

decreased