Because it's massive and alot of immigration will be in that not just the 15 billion
The biggest part of the welfare budget is the state pension:
"Social security spending in Great Britain
In 2025 to 2026 the government is forecast to spend £322.6 billion on the social security system in Great Britain. Total GB welfare spending is forecast to be 10.6% of GDP and 23.6% of the total amount the government spends in 2025 to 2026.
Around 55% of social security expenditure goes to pensioners; in 2025-26 we will spend £177.7 billion on benefits for pensioners in GB. This includes spending on the State Pension which is forecast to be £146.1 billion in 2025 to 2026.
In 2025 to 2026 we will spend £145.0 billion on working age and children welfare. This includes spending on Universal Credit and its predecessors, and non-DWP welfare spending.
In 2025 to 2026 we will spend £77.1 billion on benefits to support disabled people and people with health conditions, and £37.3 billion on housing benefits."
Source:
https://www.gov.uk/government/publi...-and-caseload-tables-information-and-guidance
This is what Asylum Seekers/irregular immigrants can get:
- What you'll get
You can ask for somewhere to live, a cash allowance or both as an asylum seeker.
- Housing
You’ll be given somewhere to live if you need it. This could be in a flat, house, hostel or bed and breakfast.
You cannot choose where you live. It’s unlikely you’ll get to live in London or south-east England.
- Cash support
You’ll usually get £49.18 for each person in your household. This will help you pay for things you need like food, clothing and toiletries.
Your allowance will be loaded onto a debit card (ASPEN card) each week. You’ll be able to use the card to get cash from a cash machine.
If your accommodation provides your meals
You’ll get £9.95 for each person in your household instead.
If you’ve been refused asylum but you’re still eligible for support
You’ll be given:
somewhere to live.
£49.18 per person on a payment card for food, clothing and toiletries (or £9.95 per person if your accommodation provides your meals).
You will not be given:
the payment card if you do not take the offer of somewhere to live.
any money.
- Extra money for mothers and young children
You’ll get extra money to buy healthy food if you’re pregnant or a mother of a child aged 3 or under. The amount you get will depend on your situation.
Your situationExtra payment per week
Pregnant mother£5.25
Baby under 1 year old£9.50
Child aged 1 to 3£5.25
Maternity payment
You can apply for a one-off £300 maternity payment if your baby is due in 11 weeks or less, or if your baby is under 6 months old.
- Healthcare
You may get free National Health Service (NHS) healthcare, such as to see a doctor or get hospital treatment.
You’ll also get:
free prescriptions for medicine
free dental care for your teeth
free eyesight tests
help paying for glasses
- Education
Your children must attend school if they are aged 5 to 17. All state schools are free and your children may be able to get free school meals.
Source:
https://www.gov.uk/asylum-support/what-youll-get
It isn't going to be that much of the welfare budget considering irregular immigration was 46k for the year ending December 2025. There were 101k asylum claims in the same year in total.
In the previous year, of 135k asylum claims given initial decisions 42% (previous year 47%) were granted.
As of Dec '25, 107k were in receipt of the above Asylum support.
www.gov.uk
Not sure that is anywhere near a lot of the welfare budget as you've suggested.