Disability-related spending has risen, but overall welfare spending is flat & similar to the level in 2015/16 

This chart shows spending on non-pensioner benefits as a share of GDP.
It includes all benefits that are paid to working-age adults (including children's benefits), excluding pensioner benefits and the Covid relief schemes (e.g. furlough). It uses official data for Britain, including the OBR's latest forecasts from the autumn 2024 budget, but presented in a different way than in the official statistics. 

This chart shows that spending on health/disability-related benefits (including things like PIP, incapacity/industrial injury, sickness benefit (in the 1980s), carer benefits and disability-related housing benefits. It shows that disability-related spending has risen over time, reaching almost 3.0% of GDP.
Notes: Years are labelled as calendar years for ease of presentation, but actually refer to financial years (so '2025' is actually the 2025/26 financial year). This includes a small share of child-focused benefits that are paid to adults above working age (where they are the parent/guardian for a relevant child). The Household Support Fund from 2021/22 is not included in the DWP tables and is therefore not included here, but will make a minimal difference to results (≈0.02% of GDP). These figures include Council Tax Benefit (£4bn in 2012/13 before being trasnferred Note that following DWP practice, benefits exclude Northern Ireland but the GDP figure is for the UK as a whole (hence the actual share of British GDP is ≈0.12-0.16% higher). 
Changes to previous versions: Council Tax Benefit has been separated out from the pre-2013/14 data, as from this point it was rolled into the general allocations to councils, who then designed their own schemes. Note that two other benefits that no longer exist are still included: Income Support in Residential Support/Nursing Homes (ends 2003/4, £1bn peak in 1998/9), and War Pensions (ends 2001/2, £600m peak) - but these barely affect the overall picture.
Sources: DWP Caseload & Expenditure tables 2024, supplemented with Scottish social security forecasts for disability benefits from 2020/21, and revisions from the OBR Economic & Fiscal Outlook October 2024 (Chart 4.8; note that I have had to assume that the 'disability' and 'UC' categories there map onto the categories used above). Data is for Britain only (i.e. excluding Northern Ireland, which falls outside DWP's remit).
Chart: Ben Geiger, 28/11/2024Source: link to DWP Caseload & Expenditure tables 2024Get the dataCreated with Datawrapper