The 1926 and 1927 races were officially the first and second Royal Automobile Club Grands Prix - mimicking the French, whose race was (at the time) called the Grand Prix de l'Automobile Club de France.
The Silverstone race in 1948 was called the RAC International Grand Prix.
The first 'RAC British Grand Prix' was the 1949 event.
You have to bear in mind that the RAC was on the horns of a dilemma at this time. The RAC Tourist Trophy had always been Britain's Grande Épreuve on the AIACR International Calendar. Grandes Épreuves were the framework on which the calendar was constructed; these were fixed and announced at the CSI's first calendar meeting in mid-September. There was a second level known as Épreuves à Priorité - GEs and EPs were so arranged as to never clash with each other; the rest of the calendar was constructed around them and announced after the official AGM during the Paris Salon the following month. In most years during the 1930s the TT and the Indianapolis 500 were the only GEs not run to the prevailing International Formula - and in 1938 the TT was the sole exception (it would also have been in 1939, had Hitler not intervened). Confusingly for our European cousins the continental press - especially the German press - often insisted on calling the TT the Grand Prix of Great Britain. You can also find references there to a United States GP, always actually meaning the 500.
In 1947, 1948 and 1949 the RAC had reserved dates for the TT as their Grande Épreuve, but in all three years they had been compelled to cancel it due to the lack of a suitable circuit. The 1950 TT was run but was not the British Grande Épreuve. When you consult the official lists of Grandes Épreuves, you find that amongst them is the Grand Prix d'Europe which was - at the time - a purely honorific title. In practice this title was always given to one of the existing Grandes Épreuves, but at least in theory it could perhaps have been conferred on any other event chosen by the CSI. But - and this is an important point - no country could host more than one Grande Épreuve. So, when the Silverstone race was announced as the Grand Prix d'Europe for 1950, that effectively forced the RAC's hand; from 1950 onwards the RAC British Grand Prix became the British Grande Épreuve, supplanting the Tourist Trophy.