The plan was to keep the 777s flying into their 30s. Boeing is unable to deliver 787s at any reliable pace and unable to certify the 777-9. RR isnt providing engines to BA’s liking for the A350, so their fleet replacement plan is stuffed. Another site mentions the 777s will be replaced in 5 years, but as BA/IAG has no firm orders beyond the 777-9s and there are long waits for A350s I cant see how the 777-200s will be replaced in any great numbers for the next 10 years.
Indeed some quarters speculate BA may take second hand A380s, such is their fleet shortage.