Unless abused by rodents, DIYers, or thermal damage I would expect 1975 PVC wire is likely to be perfectly serviceable. But as raised above, very likely you will be short of socket outlets where you want/need them.
With a new board around 15 years ago it might have RCD protection on some circuits, but that it not guaranteed. Also depending on the make/model it might be getting difficult to source any spare parts now or to have space if you need any new circuits added, so you probably should be budgeting/planning for a new board as soon as you can irrespective of any EICR before purchase.
Typically a new board today will have surge protection, and the majority of folk on this forum would be recommending and fitting a board with all RCBO so a fault on any circuit only trips that set of loads, and not multiple circuits as you get with the older/slightly cheaper single or dual RCD style of board.
So without a recent EICR I would go with the above and assume/budget for a full rewire in the 5-10k range (very much dependent on the size of the house and area) and you might be pleasently surprised. But even with an all-clear EICR set of results I would be budgeting around £1k for a new board and any minor things to be added before you decorate to get settled in.
