CGT- this really depends on how much your parents paid for it, whether they spent money on it ie enhancement expenditure, when they bought and so on. Roughly, CGT is a tax at 20% on the profit made between the time they bought and when they transfer to you ( the 'sale' price is the market value of the house when transferred to you) but they can deduct enhancement expenditure, legal and auctioneering costs, stamp duty, and so on. The net purchase cost can be indexed up wards by a specific multiplier depending on the date of purchase.
Inheritance tax- if you have had no other gift before from your parents, and you are paying 300000 for a house worth 450000, then you are getting a gift of 150000- this is below the threshold for inheritance tax for a gift from ones parents, and so, unless you have had other gifts which when added to this, puts you above the threshold ( currently466,725) there is no inheritance or gift tax payable.
How does the valuation work? And ' could they say the 150k is a donation'; I don't really understand these questions- the valuer will obviously say the house is worth 450000, but you are paying 300000 so you are getting a gift of 150000- this seems straightforward to me. You may be asked by your bank to have your parents sign a deed of confirmation that despite the gift of 150000 that they have no further interest or right to the property. This is straight forward and your solicitor will advise you.