With this type of investment, you are allowed to withdraw 5% of the original investment per policy year, hence why I asked for the exact date of investment. In any year where the 5% allowance is not fully used, the unused part can be used in a later year. You are allowed to withdraw £1,500 per policy year (£30,000 x 5%). If £2,500 had been withdrawn in year 1, there would have been a chargeable event gain of £1,000 (£2,500 - (£30,000 x 5% x 1)).
Assuming the policy is a UK policy, any chargeable event gain is treated as basic rate tax paid and there would only be tax to pay by you if you were a higher rate taxpayer in the tax year the chargeable event gain occurred. The gain would be divided by the number of complete policy years ("relevant years") which had passed since the last chargeable event and that fraction would be added to your other income to see if that took you into the higher rate tax bands. If it did, then the tax due on the part of the fraction which goes into the higher rate tax bands would be multiplied by the number of "relevant years", you would deduct the basic rate tax treated as paid on that fraction and that would give you your tax liability.
You withdrew £25,000 last year. Assuming that happened in policy year 17 and that there have been no earlier withdrawals, your accumulated unused allowance was £25,500 (£30,000 x 5% x 17). There would have been no chargeable event gain as you took out £500 less than the accumulated allowance of £25,500. If the withdrawal occurred in policy year 18, then your accumulated allowance would have been be £27,000 (£30,000 x 5% x 18) and there would have been no chargeable event gain.
You can read about blocking and spreading here and here. The idea is to reduce the potential chargeable event gain by surrendering one or more separate policies within the bond and partially surrendering the others.
If you take out £5,000 evenly across the board (spread), you will have withdrawn all the original investment before the 20 years of 5% allowances (£30,000) has been reached so there will be a gain of up to £3,000 depending on the exact date of the original investment. The gain is worked out on a completed policy year basis. If the event occurs in policy year 19, the gain will be £1,500 (£25,000 + £5,000 - (£30,000 x 5 % x 19)). You would divide the gain of £1,500 by the number of completed policy years (say 19) and the fraction of £79 (rounded up) would be added to your other income of £30,000. That will not take you into the 40% tax band and assuming the policy is a UK policy as opposed to an offshore policy, there would be no tax to pay as basic rate tax is treated as having been paid.
With offshore policies there is no basic rate tax credit so assuming you pay tax which you do, you would have a tax liability of 20% of the gain.
Given that until last year, there had been no previous withdrawals, there is little if anything to be gained by a block and spread strategy. Based on what you have told me, there will be no tax to pay unless the policy is an offshore one.
I hope this helps but let me know if you have any further questions.