do i need to pay any tax on my profit of 15 lakh.
Yes you need to pay capital gains tax. This transaction is not exempt as the gains arise in India and hence subject to tax even for NRI.
If the property is held for less than 3 years, it would be short term gain and taxed according to your income tax slab.
If held for more than 3 years, it would be long term gains at 10% without indexation or 20% with indexation.
would i be able to transfer funds from my NRO to NRE account ?
No. NRE Accounts credits are only on certain permissible transactions. Funds in general cannot be transferred from NRO to NRE.
If you had purchased the house by debiting your NRE / FCNR account, or taken a home loan and repaid this from NRE / FCNR account [essentially from foreign remittance] you can reptriate the same amount back. i.e. in you case if you had debited NRE account for 35 lacs you can get back 35 lacs in FCY.
The profits can also be repatriated under an overall limit of 1 million USD. So you need to get the funds back to UK and then send it back into your NRE account.
Refer the RBI guideline