It's not clear who has spent the Reserve - the previous government or the current one;
When Rachel Reeves pointed to a £6.4 billion in-year spending pressure on the asylum system, she was in effect pre-recognising the top-up that the Home Office was banking on receiving later in the year. The problem is that this time around, with major spending pressures elsewhere, there does not seem to be enough left in the Reserve for such a top-up.
There's £5 or £6 billion to be saved here - the article says the Conservatives were preparing to ditch the pledge;
https://www.theguardian.com/environment/article/2024/jul/26/ed-miliband-labour-honour-pledge-11bn-overseas-climate-aid
Also, I don't understand why the IFS hasn't mentioned the foreign aid budget in their analysis - surely it's a political choice on how to spend the funds;
April 2024
The Foreign Office statistics reveal £4.3bn of its foreign aid budget went on supporting refugees and asylum seekers in the UK.
That represents a 16% increase from the £3.7bn that was spent in 2022.
Much of the £4.3bn is spent by the Home Office (about £2.9bn), mostly on housing for asylum seekers. The rest covers education, health and other social needs.
https://www.bbc.co.uk/news/uk-politics-68781450