can I just point out that the OP is in the UK, in May it is normaly Quite cold and riding schools don't like you giving thier horses full baths in that weather.
In the last 2 weeks I have bathed my show pony twice and he has been shakeing with cold both times, now I have access to running hot water, a horse shower and I am extremely well practiced in the art of giving a horse a bath as quick as possible (record is 20 mins flat), I also have a mountain of rugs and duvets to put on the horse afterwards which doesnt normaly happen at riding schools.
Please Ask before you do anything to the horse perticularly when it comes to pulling manes or trimming whiskers. Lots of horses realy don't like clippers or trimmers and can react violently towards them, some people do not like you trimming the whiskers off a horse or stripping out ears. Before you start putting anything near the horse ask the owner if the horse is allergic to anything and what they do and do not want you to do to the horse
About a week before the show I would ask your instructor to show you how to pull a mane and trim the jow bone/ears (preferably on the horse you are going to use).
On the day before I would give her a proper quartering, not just a good brushing but a proper grooming. I was always taught that with any colour of horse other than a grey you do not need to bath (it makes it quicker) but a proper grooming will give the same effects.
You will need a full grooming kit including:
Rubber curry comb,
Plastic curry comb,
Metal curry comb,
Hoof oil brush,
Stable rubber (also known as a tea towel)
Piece of paper
Start with the plastic curry comb to get any mud off, then use the rubber curry comb in circles to loosen any hair or dirt off, press quite hard. Dandy brush to remove what you have raised with that.
Then contrary to popular oppinion I'm going to tell you that the metal curry comb is not actualy for use on the horse itself it is for use to clean your body brush. You should use the body brush vigarously on the horse and scrape it accross the metal curry comb every 4-6 strokes.
Every so often tap the side of the metal currycomb on to the piece of paper to dislodge the dirt then using the paper put dirt into the egg cup.
Normaly you are done with the bodybrush when you have done both sides of the horse, the egg cup is full, your arm is aching like fury and you are sweating heavily.
Once you have body brushed you should then use a stable rubber. Often this is done using just the tea towel.
At this point I normaly strap a horse but this is best not attempted unless you have had someone show you how to do it properly.
To lift more dirt up you can hot cloth the horse which involves warm water with a smidge of lavender oil in it (some people use vinegar or baby oil depending on preferance), to hot cloth you need to dip cloth in bucket of HOT water (as hot as you can stand) then wring it out so it is nearly dry and wipe against horse pressing hard. The towel will normaly come away a disgusting colour, rinse towel and repeat untill cloth comes away clean (you will need to change your water quite often). This option means that the horse doesnt get as cold as a bath and you can leave part of the horse covered in rugs.
Then pick out and scrub the horses hooves, oil them. I would also reccomend washing horses socks as generaly it is very difficult to get them clean any other way.
Use 1 sponge to clean horses bottom and anouther to clean its eyes and nose.
On the day of the show repeat the above (without the hot clothing), spray with a non greasy show shine avoiding the saddle patch (I reccomend Absorbine, greasy show shines wsill just bring dirt to the surface)
Put chalk on her socks, oil her hooves
I would suggest that you get a waterless shampoo for any stains on the day.
Above all enjoy yourself and don't obsess over things you can't control.
This pony was done using the above methods! Before these photos were taken he was NOT bathed (he could be difficult to bath so if I could get away with it I didnt bath him)
This show was indoors so quite early in the year I think!
This one was in late May, the weather was miserable all day the previous day and on the day of the show we had thunder, lightening and a hail storm! I was very lucky to get my class in the only dry patch of the day.