If you go down the IDP route to save the £10 to update you photocard licence be aware that there are two different types of IDP of which some countries need one type and other countries the second type. IISMC Spain for instance demands an IDP in addition to your UK licence.
Be aware that technically you also need a UK sticker for your vehicle even if you have a Union flag and the UK letters on your registration plates. The stick should be 18x13cm which IMO is unnecessarily large and ugly but that's UK regs for you!
Yes, there were two international treaties on road transport which had provisions for mutual recognition of driving licences, one from 1949, the other 1968. Spain signed the 1949 one, most other countries the 1968 version. The UK signed both although they only ratified the 1968 version which made it easier for the EU to accept UK licences without the need for an IDP in 2018, just in time for Brexit. Because the UK signed both treaties some bright spark believed that it was necessary to issue both versions of the IDP, but countries that only signed the 1968 treaty will only issue IDPs according to the 1968 rules. Therefore, there is really no need for the 1949 version any more even though Spain only signed that one. (There's no need for Spain to follow by ratifying the 1968 agreement so long as they are in the EU because EU rules already provide for mutual recognition).
The IDP is
always valid
only in conjunction with your national driving licence. Spain is no exception. The whole purpose of an IDP is to give foreign authorities an indication of what classes of vehicle one is allowed to drive if they can't read or interpret the national driving licence. The 1949 treaty is not quite so clear in this respect. That's another reason not to bother with issuing a 1949 IDP.
There is an EU/UK agreement that number plates with the Union Flag and the letters UK do not need a separate UK sticker, except in Malta and Cyprus for some strange reason. The same applies to Norway, Iceland, Switzerland and Liechtenstein which are also not in the EU. The size of the sticker is regulated by the UN, not by the UK.