1993 XJs were usually R12. Look at the service valves. Take th edust cap off one. If the valve is threaded on the outside it's an R12. There should also be an information sticler somewhere under the hood.
A proper retro fit 134a conversion and involves replacing the hoses and valves, flushing the system replacing all the lubricant before recharging, but this costs hundreds. The cheapest way is to remove the old R12, inject a special additive that makes the original lubricant compatible with R134a, put adaptors on the original valves and recharge to 80% of the original capacity with 134a. Proper A/C specialists frown on this method but it does work.
£90 is cheap anyway.
