The NMW increase in April has no bearing on your furlough pay if you have been continuously furloughed since before April.
Guidance below is:
Your employer will receive a grant to cover up to 80% of your wages up to a maximum of £2,500 per month, which they should then pass on in full to you (they can top it up if they wish, but they do not need to do this).
If you are a salaried employee who receives regular pay, they should claim a grant of 80% of your salary as at 28 February 2020, subject to the £2,500 cap. This means that if your gross (before tax) pay in February was £1,231.50 (based on your contracted hours x £8.21), then in April 2020, your employer will receive a grant of 80% of £1,231.50, which is £985.20.
If your pay varies, AND you have been in the job for over a year then, subject to the £2,500 cap, your employer can claim the higher of:
80% of the same month’s earning from the previous year
80% of average monthly earnings from the 2019/20 tax year
If this applies, this means that what your employer can claim will be based on what you were paid in April 2019, or on your pay for the 2019/20 tax year – both of which will have presumably been based on the old £8.21 minimum wage rate.
All of this means, essentially, that you won't get the pay rise that you may have been expecting. However, when your period of furlough ends, and you return to work, then you should be paid the £8.72 rate from that point.
Note, that being paid less than £8.72 during the period of furlough, does not mean that you will have been underpaid the minimum wage because you are not actually working, click here to read this government guidance explanation.