Tax credits are used to reduce your PAYE tax liability and they are not refundable nor can they be carried forward so if you don't have enough taxable income to use them then you lose them.
What do you mean by "end of year tax credits"? For PAYE workers normally your credits (and standard rate cut off point and tax rates/bands) are issued at the start of the year (and reissued during the year if your circumstances change) and then applied at payroll. This is the purpose of your statement of tax credits. Some credits/reliefs can only be claimed after the tax year has ended - e.g. medical expenses relief.
So what are you asking then? If you have medical/dental expenses during the year then after the tax year ends you can file a MED1/2 return and claim tax relief (at standard rate) on any qualifying expenses that are not otherwise reimbursed (e.g. by insurance etc.).
T Mcgibney , thanks for the link. Clubman, you brought up financial engineering, thats outrageous and cynical talk. I was completely clueless about this topic and now im not. Thanks everyone!
Huh? No it's not. My point is that with most common PAYE related credits/allowances you just get/claim them or not and there's generally no real room for creativity in case you or anybody thought that there might be.