The enforcement of fines by the courts has many stages too lengthy to go into here. In short, the court will go to considerable lengths to avoid sending a defaulter to prison.
It is important to note that your financial circumstances would have been taken into account when the fine was assessed. If your circumstances have changed since then you should tell the court and there is a possibility that some or all of the fine may be written off.
The first thing the court will want to establish is whether the lack of payment was because of genuine lack of funds or because of wilful refusal to pay. If it is established that you do have the funds there are many options open to the court to get the fines paid. Attachment to earnings or deduction from benefits orders, or a distress warrant (seizure of goods) are examples.
Only if it is established that there is wilful refusal to pay and other efforts have failed to secure payment will prison be considered. The”tariff” for a fine of £800 is 28 days and the sentence is not subject to the 50% discount as normal sentences are. If it gets to this stage the court will almost certainly suspend the sentence in the first instance with an order for payment (either in full or by instalments as they think fit).