What sort of travel costs can i claim back with a NHS Tax Credit Exemption Certificate?

i will need to go for specialised exams and appointments in hospital and this is 3 bus away and takes me 2 hous almost, i get there feeling even more sick and tired (i have lupus) would it be ok to get a taxi? the difference is only 5 pounds and it would make a world of difference has i would not waste my day or 4 hours traveling ,the taxi takes 25 minutes.