There is no honest single number for custom business software in the UK. A quoting tool for one sales team is not the same project as a customer platform with payments, accounts and administration.
What you can do is bound the work. Describe the users, the jobs they complete, and what “done” looks like in the first release. The first release should solve a real bottleneck, not every future idea.
Budgets we typically see for a first useful system sit in bands: a tightly scoped internal application; a website or e-commerce build with real functionality; a longer platform that will keep growing. The enquiry form on this site uses those bands so we can talk in the same range.
Ongoing cost matters as much as build cost. Software that cannot be changed will be replaced. We design for further development so the first version is an asset, not a dead end.
If you want a conversation grounded in your process rather than a catalogue price, start a project and tell us what is slow, duplicated or stuck in a spreadsheet. We will say whether the right answer is software, a website, an integration, or not building at all.