A UK company costs < £100/yr to maintain
Accounting included ?Off the top of my head, the Companies House filing fee of around £15/yr, an address for the company (around £50/yr) are all you require to hold a UK company.
For a tech company that uses Xero and feeds in invoices programmatically filing the statutory returns by an accountant might cost £1000/yr (or 0 if you wish to DIY, the UK's HMRC makes self-filing easy unlike the IRS). Plus £360/yr for Xero (free accounting software probably won't cut it).
VAT returns and PAYE returns are pretty easy to self-file; Xero does them automatically and HMRC provides free software for it. The annual statutory accounts are more difficult, and you probably want an accountant.
Rough figures: if you want to be cheap, use Xero discounts and DIY, it's around £500/yr. With an accountant, but you still doing some of the work, perhaps £1500/yr.
Depending on your volume, it's really not expensive to setup a legal entity in the UK. Since UK is leaving (unsure if EU financial access will continue) Ireland is a good alternative. A UK company costs < £100/yr to maintain. Depending on your volume, it may be worth exploring with your lawyer/accountant.
Perhaps there's merchant-of-record services too, that are incorporated in the EU and I believe legally act as an agent to sell your product.