If you realize that your application contains a clerical error (sufficiently material, which in your case it is) the first remedy is to contact the consulate (or Visa Facilitation Centre) where you submitted it. You can do this by email or fax or by post. The critical information in this (and any communication to a British visa issuing post) would be:
- your name
- date of birth
- passport number
- application date
- application number.
Consular staff will not alter your application based upon a late communication, instead they will do one of the following...
- attach a note in Proviso citing your email and if they determine that the mistake is trivial, they will proceed to the decision-making stage;
- reject the application, to allow you to correct the error and
- if they have already made a decision and they determine that the same decision would have been reached with or without the correction, then they would do nothing. The decision would stand and they will not reconsider.
- if they have already made a decision and they think the error would have changed their decision, they will cancel your visa
If you think the error is likely to be significant and/or raise credibility problems, you should request that the application be withdrawn.
If the fee has not been yet been collected, you have the additional option of blocking the payment. This will cause your application to be set aside as incomplete. You can then withdraw the application, fix the error(s), and resubmit it. It commonly happens when someone's credit card company bounces a payment.
This is covered in Paragraph 30 of the Immigration Rules...
An application for an entry clearance is not made until any fee
required to be paid under the Consular Fees Act 1980 (including any
Regulations or Orders made under that Act) has been paid.
Also Paragraph 34T...
A notice informing an applicant that their application is invalid will
be given in writing (which includes, where an email address has been
provided for correspondence, by electronic mail).
Note: a rejection is always without prejudice, it is not recorded in the same way a refusal is; and there is no bad history created when an application is rejected.
Based upon what you wrote, you ticked the NO box on Section 6.2 "Have you travelled outside your country of residence, excluding to the UK, in the last 10 years?" If you filled out the rest of Section 6.2 they are likely to treat it as a minor clerical error. On the other hand, if you left the entire section blank, then you should press for a withdrawal or rejection. They would not be happy seeing an entire section omitted because that's hard to explain as a trivial clerical error.