@onepict 150billion if wages go up a lot. Otherwise I'd say UBI would probably cost about 200-250 billion annually at first, in the UK.
For reference, current government income is about £1 trillion. Some taxes would go up at first, but we can expect more people would start businesses if they have more time / less stress due to the support. So GDP goes up.
The rest of the money can be found be increasing productivity in the public sector, especially healthcare.
UBI is quite feasible in the UK.
@onepict I mention healthcare because it's a huge percentage of government spending, in fact it's even been on the news recently. With UBI, more people would live at a higher standard, especially in terms of diet and exercise, which would result in fewer illnesses in the first place.
You could probably implement UBI without even increasing taxes at all, if I'm being honest, but any such rises would be temporary; a lot of people already get certain benefits anyway (in-work e.g. child tax credit)