im sure Barlow trailers was about £150 a day last time we hired a few years back.
We've hired from Marie at Sorrel Court Horseboxes (on Facebook and also Google), she's in Yorkshire, She has a 3.5 Tonne Foxy stallion box and it is heavenly to drive, CCTV and a really decent box and was very economical on fuel (we did an Essex/Yorkshire run), extremely easy hire process and if the box is in you can collect earlier for no charge).
On facebook EQUOS Barnsley Yorkshire based driven or self drive 3.5 ton sometimes a discount if returning empty from a delivery . ORLhorsebox hire Sheffield Yorkshire based self drive £90 a day. CMSTrailers Huddersfield Yorkshire hire a selection off I for Williams trailers £50 a day (last yr price probably still same)
Barlows have gone up in price recently to £130 for the day but have used them quite a bit and never had any problems. Ainscoughs have just started up and quite often have deals on - they’re £110 per day. Not used them but know a few people who have http://www.ainscoughhire.co.uk/horse_box_hire