1. Brexit has reduced the size of the UK economy.
2. COVID cost the country untold billions. In part to keep much of the population in idleness for 18 months. Another big chunk went to Tory mates for supplying useless kit. And several 'businesses' simply stole money. And we are not talking £50 either.
3. The Ukraine war has severely damaged the whole European economy. In addition, we are paying out huge subsidies to keep Ukraine going.
I think these things together explain most of the problem. And I don't see any magic solutions. Cutting tax is unrealistic, so if you want to cut Tax A, you have to increase Tax B. That's reality.
The fuel tax has been frozen for a long time. Realistically, that needs to end. Sorry, motorists, but while you've enjoyed your freeze, no one froze train fares.
I don't think you can hope to pull all the necessary money from the rich. The honest way is to increase income tax for everyone and have a massive crackdown on tax dodging. I would also introduce Land Value Tax. The landowners can't take their land out of the country. But, first, I'd freeze all rents indefinitely, including agricultural rents.