Next time you post the question before you act. What you did was utterly, utterly stupid. You will lose 45% income tax.
Go to a good tax advisor to see if the situation can be salvaged. If he charges you £5,000 to fix it, thats dirt cheap compared to paying £150,000 in taxes. Normally 25% of cashed in pension are tax free but some forms need to be filled out before taking the money. Hopefully that’s fixable. Then the next £37,700 a year cost 20% income tax. So all you had to do was spread the cash withdrawal over several years. If that can be fixed you’d have to return the money.
But there is a worse case: A scammer making you invest your pension in a fake investment fund. So you are promised 20% if you invest your £300,000 with them. The scammer told you no tax would be paid to get your money. With your money, he does a runner. So your pension money is gone. HMRC doesnt care that your money went to a scammer, they want your taxes, so now your pension is gone and you owe HMRC. (One police officer lost his complete pension fund to a scammer and now owes HMRC £150,000, that was quite widely publicised.)