Costs of a UK will using a solicitor vary enormously depending upon the solicitor concerned and the complexity of the estate. A rough figure might be between GBP 500 and GBP 1,000.
As for your Thai estate, you would be intestate. I believe that stepchildren are not on the list of statutory heirs. Your wife (assuming you have a legal marriage, not a village or temple one) would inherit your Thai estate. However, better to make a separate will covering your Thai assets to make things absolutely clear.