Sell your power meter to fund some aero kit!
(If you don't have a PM, where did that 260 W figure come from? A Wattbike? I hope you aren't being naughty and trusting Strava's estimated power figures? They're a complete work of fiction.)
Things you can do for little or no money:
1) You have tribars, right? Move them closer together. Your hands should be touching, and your elbows not all that much further apart. At least as narrow as mine - I carry a bottle between my arms as I mostly do longer events, but I'd investigate different water storage options and arms closer together if I didn't have too much invested in my current setup, and too few TTs planned in the coming season, to make a change worthwhile.
2) If you have a different saddle position for road and TT, get a second saddle and a seatpost to go with it and mark both of their insertion points with tape, so you can swap them quickly. If you can stretch to one, a specific TT one like the ISM Adamo is more comfortable than a standard road saddle.
3) Did I say a different position? Don't trust the 'up and forward round the BB' position advice: it works for some people but it's outdated and often wrong. A lot of people, myself included, find it's more aero to drop your saddle *down* a bit and then move it backwards until you've got enough height. You'll lose a bit of power but it will come back with training in position, and the aero gains are worth it. If you're usually a toes down pedaller, learn to be a heels down pedaller.
4) Practice dropping your head and looking out of the tops of your eyes. It'll be uncomfortable at first but again, gets more natural with training in position. The standard advice is to concentrate on pushing your chin both down and forward.
5) Once you've got that position, train in it! Some core and neck exercises off the bike will also help.
6) Get a helmet. The Giro Selector and Bell Javelin are a couple of generations old now but are known to test reasonably well on a wide range of people, and can be picked up dirt cheap secondhand.
7) Get a skinsuit. The
Velotec ones are comparatively inexpensive (£120) compared to other good ones, but have been spotted on some very good and very attentive riders so must test well.
8.) Do you wear mitts to TT? Don't.
9) Do your shoes have buckles poking out? Get some oversocks. The shiny Madison ones are about £15 but are meant to be a good budget option - though beware, I've got some and you need to size down.
10) Shave your legs
11) Where are your gear and brake cables? Go to any lengths you can to hide them from the wind.
10) How wide are your tyres? Get whatever width will make the tyre shape flow smoothly into your rims. Get some
low rolling resistance ones - the Continental GP4000/5000 is a popular choice for a set with a bit of puncture resistance.
11) Remove any bottles from your frame before you TT. If you must have a bottle, the ST one causes less drag than the DT one, but if you're doing a 10/25 you can remove both.
12) Your drivetrain should of course be absolutely clean, lubed and shiny.
Hopefully that's enough to be getting on with