As previously said, your room is quite large and one single 4ft fitting isn't enough. You cannot make the existing fitting any brighter. Rewiring it for an LED tube would not yield a significant lux level increase, if any at all, in fact it'd more likely be lower. You need to replace the fitting for something new.
In an ideal world you need more fittings with diffusers/louvres to provide a glare
free & even illumination. In the real world you may not want to start rewiring this room so replacing the existing fitting with something more suitable fitting may be the way to go. You could also add a angle poise light or similar to your desk if needed.
I'd recommend a twin T5 '5ft' (1.45m) fitting with cat 2 louvre, in either 'High Efficiency'(HE) 35w or 'High Output'(HO) 49w. Something similar to this, note these are usually around £80 for an electrical wholesaler:
https://www.ebay.co.uk/itm/323595589155