I am certainly not qualified to give legal advice, this is just my understanding of the information you have provided
The only reason both roles are lower pay per month than what you currently take home is because of your commission, which they could say is not part of the equation because it is effectively a bonus because of how well you do the job - if you weren't as good at the job you could be taking home just the £25K per year and they have offered you another role at that same figure
You do say that you could potentially get back to that £45K figure with the first job, but it would take time to build the relationships and increase your commission, which is what you appear to have done in your current position to get to the figure you are at now
Unfortunately, whenever commission is involved, you can't really take it as guaranteed income, as a bad year could see all of that wiped out in a worst case scenario, but I understand where you are coming from in terms of what you currently take home and what you have been offered
Have you based your lifestyle/spending on the £45K, or could you live on the £25K or £35K if you took one of those roles?
Logic would tend to dictate you take the higher paid role to minimise the instant loss of earnings, but I would be keen to know if there would be scope for progression with increased pay