that's the reason i asked how long you would need it for and if your stairs are straight. Where i live you can rent them for straight stairs at a cost of £450 a year (installed). If you don't anticipate needing it for a long long time, it is much cheaper
Acorn make a lot of kit which other firms buy and use. With firms like Stannah the fitters are often locally based and some not so good as others. You can get reconditioned ones cheaper and they often have had little use.
Age UK might be more affordable.
Above all, beware of the sales tricks that the larger firms sometimes employ. We got huge reduction on an electric bed by waiting till the cooling off period was nearly over and phoning to "cancel". They pulled out all the stops to complete the sale
If it is considered necessary for your wellbeing apply to the local council for a grant. My stepfather got grants for both a stairlift and a downstairs toilet to make life easier for him.