Hi Steve = section 9.1 of
PG 67 confirms that your identity can be "
verified by someone in, or who before retiring worked in, one of the authorised professions below" - the list includes a Police Officer
There is always a cost to using professional services. However the death is a very sad fact so if you do nothing then the legal ownership remains as part of your late Mother's estate. No law is being broken as a result and the only possible 'challenge' in such cases would tend to be from the family group for example re the executor not carrying out their responsibilities as an executor in dealing fully with the estate.
Registration fees can be paid by cheque or by postal order to HM Land Registry. The payment of fees by other means is being developed but as yet not implemented.