Harry Dunn’s parents have said they “couldn’t be more proud that Harry’s given us the strength to do it” following their High Court battle with the Foreign Office.

Mr Dunn, 19, was killed when his motorbike crashed into a car being driven on the wrong side of the road by American Anne Sacoolas outside RAF Croughton in Northamptonshire on August 27 last year.

Sacoolas, whose husband Jonathan Sacoolas worked as a technical assistant at the base, left the country a few weeks later after the US said she was entitled to diplomatic immunity.

The 43-year-old was ultimately charged with causing death by dangerous driving last December, but an extradition request was rejected by the US State Department in January – a decision it later described as “final”.

Mr Dunn’s parents, Charlotte Charles and Tim Dunn, claim the Foreign, Commonwealth and Development Office (FCDO) wrongly decided Sacoolas had diplomatic immunity and unlawfully obstructed Northamptonshire Police’s investigation into their son’s death by keeping the force “in the dark”.

After the conclusion of a two-day hearing on Thursday, Mrs Charles told the PA news agency: “We have to wait for possibly a few weeks for the outcome. But it’s done, it’s dusted.

“We feel really proud of ourselves for having brought this judicial review.

“I think it’s opened our eyes up to an awful lot that we didn’t know.

“It went as well as we possibly could have expected but we’re not lawyers, we’re not judges.”

She added: “I just feel as a family, as parents, we couldn’t be more proud that Harry’s given us the strength to do it.”

Mr Dunn told PA: “I must admit, before the last couple of days it’s the lowest I’ve felt. It’s been a real struggle.

“I thought the last two days were very informative … but it’s just a shame that Harry had to die for us to get where we are today.

“We’ve lost a lovely boy and the fight has been horrendous and we’ve had a great team help us out. But it still hurts to think that we’ve lost him to have to show what’s going on in the country and what is happening.

“Fingers crossed it goes our way when they announce their decision.”

Asked if Harry would be proud of what they had achieved so far, Mr Dunn said: “Of course, so proud.

“But he’d be saying to us: ‘Don’t you stop now – you carry on’.

“We found something we believe is not right and it needs to be fixed and Harry would be saying: ‘Come on, let’s get the right result’.”

Mrs Charles added: “He wouldn’t stop ever. So how can we?”

Their case centres on a 1995 agreement between the UK and the US, granting immunity to administrative and technical staff at RAF Croughton, which the US waived in relation to “acts performed outside the course of their duties”.

The FCDO says that waiver only applied to staff at RAF Croughton and not their family members, meaning Sacoolas did have immunity at the time of the crash.

But, on Wednesday, Sam Wordsworth QC, representing Ms Charles and Mr Dunn, said Sacoolas had “no duties at all” at the base and therefore “never had any relevant immunity for the US to waive”.

Geoffrey Robertson QC, also representing Mrs Charles and Mr Dunn, told the court the FCDO “took upon itself the authority to resolve the question of immunity and ultimately and unlawfully decided to accept the US embassy’s decision that Anne Sacoolas had immunity”.

He said that decision “obstructed the police by preventing any effective further progress in its investigation into Harry’s death and likely prosecution of Anne Sacoolas”.

Mr Robertson also argued the FCDO “tacitly accepted the Sacoolas family’s departure from the UK”, but the FCDO’s barrister Sir James Eadie QC said officials had “objected in strong terms” to Sacoolas leaving the UK.

Sir James said in written submissions that FCDO officials “repeatedly emphasised” that the department “wanted the Sacoolas family to co-operate with the UK authorities”.

He argued: “As a matter of international and domestic law, Mrs Sacoolas automatically had diplomatic immunity as the spouse of a member of the administrative and technical staff of the US mission.”

Sir James said the US “expressly waived the immunity from the UK’s criminal jurisdiction of ‘employees’ or ‘staff members’”, but “at no point is there a waiver of the immunity enjoyed by the families of such individuals”.

In July, Foreign Secretary Dominic Raab announced that “the US waiver of immunity from criminal jurisdiction is now expressly extended to the family members of US staff” at the base, “ending the anomaly in the previous arrangements”.

Sir James told the court there was “nothing inconsistent between the clear effect” of the 1995 agreement and Mr Raab “acknowledging that the effect of the arrangements, correctly analysed, was anomalous”.

In a witness statement previously before the court, Hugo Shorter, the FCDO’s director for the Americas, said officials thought the US was “exploiting the wording” of the agreement shortly after Mr Dunn’s death. But he added that “there was nothing that the UK could do to prevent (Sacoolas’) return to the US”.

Mrs Charles and Mr Dunn initially also took legal action against Northamptonshire Police but that claim was dropped in July, with the family’s spokesman saying the force had been “absolved of any blame”.

At the end of the hearing, Lord Justice Flaux, sitting with Mr Justice Saini, said they would hand down their judgment “as soon as we possibly can”.