If you are looking for a projector that you can place extremely close to the screen or wall, the amount of available projectors in the ‘affordable’ segment is unfortunately rather limited. Of the three tested ultra short throw models, two are mainly meant for business use.
The Acer U5520B can only be hung (for example from the ceiling), while the Optoma GT5000 can also be used standing. Both produce a lot of light, but do not have a great colour quality. For regular consumers we would recommend the Optoma GT5000 because it is more flexible.
The LG PF1000U is truly meant for consumers. This compact projector uses LED-lighting and therefore does not stand out in terms of brightness; the contrast is slightly disappointing as well unfortunately. Because the colour quality is certainly not perfect either, we cannot recommend this model without a doubt.
The only projector that made use truly enthusiastic is the BenQ W1210ST. This projector scores well in every regard, but: it is not an ultra short throw model. The W12010ST needs slightly more distance to the screen that the other three, albeit a lot less than a regular projector. Do you not mind placing the projector in front of you on the coffee table instead of directly against the wall? In that case, the BenQ W1210ST is by far the best choice for you.
However, this model is clearly more expensive than a comparable long throw projector. For about 224 pounds / 250 euros less, BenQ offers a model with comparable image quality that has to placed slightly further away. Therefore you should definitely consider how you want to use the projector, before you pull the trigger on the purchase of a(n) (ultra) short throw model.