Check the quote which should state which battery is supplied, then one of the general suppliers, itstechnology, bimble solar etc.
A battery installation should be zero vat, so a single 4.8kWh US5000 Pylontech battery is like £720 ex vat These are 15 cell batteries and they are essentially plug and play. Some batteries are 16 cell batteries so have a little more power stored but seem to cost a lot more. Can't mix and match batteries of different numbers of cells.
The batteries should essentially be supplied at cost, there is zero work to install more.
Decent price TBH. Should pay back in 5 to 8 years with the bigger battery and the right tariff (I think the best tariff is Intelligent Octopus Go and the fixed export tariff, ~15p/kWh export, but 7p/kWh cheap rate import, and need an EV)