I suspect that your engine needs cleaning. Start with the wheels & check the bronze wiper pickups as well (maybe wire pickups if its really old). Check that the carbon brushes of the motor are in good condition & that the brush spring & isolation sleeve are positioned correctly. Lubricate the axles & gears, careful with the axles because they are part of the electrical circuit & over lubricating can insulate them from the chassis (loco will stop & start & generally fart around). If the loco is REALLY old the flanges maybe a liitle too deep for the track & the loco will bump along on the sleepers (not so easy to fix).
I've got a number of X04 powred loco's & looked after & cleaned they are very reliable & run quite well, although they do get along at a fair clip, precise control is not a strong point of the old models, but they do run reasonably smoothly.
Worst case is that the motor has a shorted or open circuit winding & that will require replacement of the motor or a rewind of the armature, a couple of guy's advertise in the mags for motor repair like rewinds & remagnetising but try all the other stuff first...good luck.
Triang locos always run better if you remove the carbon deposits that build up in the commutator slots. Just run a pin through the slots, there are 3 as the XO4 is a 3 pole motor. Take care not to scratch the copper commutator. Then hold a tissue against the commutator and rotate the armature to remove any carbon on the surface of the commutator. Triang recommended owners did this periodically in their "user manuals". The locos run much better after this treatment. Do it whenever the loco seems to be running below par.
Make sure the felt oil retaining pads, one next to the worm, the other between the armature and the magnet, have been oiled. This doesn't apply to the motor bogies used in the Class 31, Class 37, Hymek, EM2, as they ran in nylon sleeves.
If you think you've over-oiled it, wipe off the excess with a tissue.
Also if the loco has magnadhesion check that the wheels aren't binding in the chassis. This may not apply to your Battle of Britain, but it does affect the diesel & electric locos with the motor bogie and magnadhesion, e.g. Class 31, Class 37, Hymek, EM2, etc. If it is binding you can shim out the axle concerned to give it a new lease of life.
Check that the chassis isn't knackered by checking that there isn't too much play in the wheels when you rock them up and down and side to side. If there is then you'll be looking for a low mileage chassis on a battered body at a swapmeet.
You can also replace the existing magnet with one of the more powerful Neo magnets. Locos are reputed to run better and faster with this conversion, but the one that I've done doesn't seem to have a higher top speed, just more free revving at part open throttle settings.
With care they'll last for years, possibly outlasting the newer offerings from Hornby, Bachmann etc.
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