As DarthBling says, the cash gift doesn't need to be disclosed unless you die within 7 years. It's a Potentially Exempt Transfer (PET) which becomes exempt after 7 years. However, you should make a record of the gift somewhere your executors can find (less of an issue if your children are your executors).
The apartment will be a disposal for capital gains tax purposes. As it's a gift to a connected party it will be treated as if you'd sold it to them for market value. Assuming the valuation is reasonable you can use this for the basis of your capital gains tax calculation, although HMRC prefer a surveyor's valuation and potentially could seek to challenge the valuation it's often not cost effective to obtain one for smaller transactions. This disposal does need to go on your tax return if you made a £45k profit, although if it was jointly owned you'll get to also deduct your annual exemption each, which will reduce the taxable gain.
You should also make a note of this as well, as it will need to be included in your estate if you die within 7 years. The other thing to bear in mind is that if this is a holiday apartment and you use it personally then it may not be effective for IHT purposes as you will have retained a benefit. If your kids use it and you occasionally visit them for lunch or to stay the night then that's probably OK, but staying there for a week or on your own is likely to result in it remaining taxable. You can pay your kids rent and this will negate the benefit (although this will be taxable income for the kids).